vxlan: Stop rejecting dynamic FDB entries with a nexthop ID

Commit 98fc57d16744 ("vxlan: reject dynamic fdb entries that reference a
nexthop id") started rejecting FDB entries that point to FDB nexthops
unless they are configured as "permanent" or "static". The intent was to
prevent such entries from being aged out by vxlan_cleanup(), which runs
without RTNL and can therefore race with other VXLAN devices that modify
the same per-nexthop FDB list.

However, FRR installs remote MACs that are behind a multihomed Ethernet
Segment (EVPN-MH) as dynamic FDB entries that point to an FDB nexthop
group. Since the above commit, these entries are rejected and traffic
towards the corresponding MACs is flooded to all the remote VTEPs.

These entries are also marked as externally learned ("extern_learn")
and are therefore skipped by vxlan_cleanup(), but exempting externally
learned entries from the checks is not enough: vxlan_cleanup() reads
the state and flags of an entry without the hash lock, so an entry that
is concurrently changed from dynamic and externally learned to static
can still be aged out.

Instead, remove the checks added by the above commit and make
vxlan_cleanup() skip FDB entries that point to FDB nexthops. Such
entries can only be configured from user space, cannot be learned and
cannot roam, so there is no reason to age them out. The nexthop pointer
of an FDB entry is set before the entry is linked and is only cleared
after it was unlinked and an RCU grace period has passed, so it can be
checked without the hash lock.

Adjust the test cases added by the above commit and add a test case
that checks that dynamic FDB entries that point to FDB nexthops are not
aged out. Without the fix:

 # ./fib_nexthops.sh -t "ipv4_fdb_grp_fcnal ipv6_fdb_grp_fcnal"

 IPv4 fdb groups functional
 --------------------------
 [...]
 TEST: Fdb mac add with nexthop group and dynamic state              [FAIL]
 TEST: Fdb mac add with nexthop group and extern_learn               [FAIL]
 TEST: Fdb mac without nexthop group aged out                        [ OK ]
 TEST: Fdb mac with nexthop group not aged out                       [FAIL]
 [...]

 IPv6 fdb groups functional
 --------------------------
 [...]
 TEST: Fdb mac add with nexthop group and dynamic state              [FAIL]
 TEST: Fdb mac add with nexthop group and extern_learn               [FAIL]
 TEST: Fdb mac without nexthop group aged out                        [ OK ]
 TEST: Fdb mac with nexthop group not aged out                       [FAIL]
 [...]

 Tests passed:  44
 Tests failed:   6
 Tests skipped:  0

With the fix:

 # ./fib_nexthops.sh -t "ipv4_fdb_grp_fcnal ipv6_fdb_grp_fcnal"

 IPv4 fdb groups functional
 --------------------------
 [...]
 TEST: Fdb mac add with nexthop group and dynamic state              [ OK ]
 TEST: Fdb mac add with nexthop group and extern_learn               [ OK ]
 TEST: Fdb mac without nexthop group aged out                        [ OK ]
 TEST: Fdb mac with nexthop group not aged out                       [ OK ]
 [...]

 IPv6 fdb groups functional
 --------------------------
 [...]
 TEST: Fdb mac add with nexthop group and dynamic state              [ OK ]
 TEST: Fdb mac add with nexthop group and extern_learn               [ OK ]
 TEST: Fdb mac without nexthop group aged out                        [ OK ]
 TEST: Fdb mac with nexthop group not aged out                       [ OK ]
 [...]

 Tests passed:  50
 Tests failed:   0
 Tests skipped:  0

Fixes: 1274e1cc4226 ("vxlan: ecmp support for mac fdb entries")
Fixes: 98fc57d16744 ("vxlan: reject dynamic fdb entries that reference a nexthop id")
Cc: stable@vger.kernel.org
Reported-by: Sascha Frey <sf@techfak.net>
Closes: https://lore.kernel.org/netdev/20261028142928.GA8406@techfak.de/
Tested-by: Sascha Frey <sf@techfak.net>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20261030133750.28668-1-idosch@nvidia.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>