afs: Fix cell removal

Fix cell removal by inserting a more final state than AFS_CELL_FAILED that
indicates that the cell has been unpublished in case the manager is already
requeued and will go through again.  The new AFS_CELL_REMOVED state will
just immediately leave the manager function.

Going through a second time in the AFS_CELL_FAILED state will cause it to
try to remove the cell again, potentially leading to the proc list being
removed.

Fixes: 989782dc ("afs: Overhaul cell database management")
Reported-by:  <syzbot+b994ecf2b023f14832c1@syzkaller.appspotmail.com>
Reported-by:  <syzbot+0e0db88e1eb44a91ae8d@syzkaller.appspotmail.com>
Reported-by:  <syzbot+2d0585e5efcd43d113c2@syzkaller.appspotmail.com>
Reported-by:  <syzbot+1ecc2f9d3387f1d79d42@syzkaller.appspotmail.com>
Reported-by:  <syzbot+18d51774588492bf3f69@syzkaller.appspotmail.com>
Reported-by:  <syzbot+a5e4946b04d6ca8fa5f3@syzkaller.appspotmail.com>
Suggested-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Hillf Danton <hdanton@sina.com>