CVE-2026-74427: afs: Fix netns teardown to cancel the preallocation charger

In the Linux kernel, the following vulnerability has been resolved:

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 afschargepreallocation() skip charging and make afsrxnewcall() avoid requeuing the charger.

(This was found by AI review).