bridge: check llc_mac_hdr_init() return value in br_send_bpdu()

If llc_mac_hdr_init() fails (for instance if the port device type does
not support LLC or dev_hard_header() fails), br_send_bpdu() should drop
the skb instead of resetting the mac header to the LLC payload and
transmitting a malformed frame.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Closes: https://lore.kernel.org/netdev/179022851638.2160803.1808206741379444999@kernel.org/
Cc: Nikolay Aleksandrov <razor@blackwall.org>
Cc: Ido Schimmel <idosch@nvidia.com>
Cc: bridge@lists.linux.dev
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Link: https://patch.msgid.link/20260924082951.1599377-3-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>