afs: Fix vllist leak

afs: Fix vllist leak

Fix a leak of the new vllist in afs_update_cell() in the event that it is an
empty list (nr_servers == 0), in which case the old list isn't displaced
unless the old list is also empty.

Fixes: d5c32c89b208 ("afs: Fix cell DNS lookup")
Closes: https://sashiko.dev/#/patchset/20260609081738.770127-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260622090856.2746629-18-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>