afs: Fix netns teardown to cancel the preallocation charger

Fix the teardown of an afs network namespace to make sure it cancels the
work item that keeps the preallocated rxrpc call/conn/peer queue charged
before incoming calls are disabled (i.e. listen 0).

Also, if net->live is false because the afs netns is being deleted, make
afs_charge_preallocation() skip charging and make afs_rx_new_call() avoid
requeuing the charger.

(This was found by AI review).

Fixes: 00e907127e6f ("rxrpc: Preallocate peers, conns and calls for incoming service requests")
Reported-by: Simon Horman <horms@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Li Daming <d4n.for.sec@gmail.com>
cc: Ren Wei <n05ec@lzu.edu.cn>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org
cc: stable@kernel.org