afs: Fix overwriting of result of DNS query

In afs_update_cell(), ret is the result of the DNS lookup and the errors
are to be handled by a switch - however, the value gets clobbered in
between by setting it to -ENOMEM in case afs_alloc_vlserver_list()
fails.

Fix this by moving the setting of -ENOMEM into the error handling for
OOM failure.  Further, only do it if we don't have an alternative error
to return.

Found by Linux Verification Center (linuxtesting.org) with SVACE.  Based
on a patch from Anastasia Belova [1].

Fixes: d5c32c89 ("afs: Fix cell DNS lookup")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeffrey Altman <jaltman@auristor.com>
cc: Anastasia Belova <abelova@astralinux.ru>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: lvc-project@linuxtesting.org
Link: https://lore.kernel.org/r/20231221085849.1463-1-abelova@astralinux.ru/ [1]
Link: https://lore.kernel.org/r/1700862.1703168632@warthog.procyon.org.uk/

 # v1
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>