afs: Fix uninit var in afs_alloc_anon_key()

Fix an uninitialised variable (key) in afs_alloc_anon_key() by setting it
to cell->anonymous_key. Without this change, the error check may return a
false failure with a bad error number.

Most of the time this is unlikely to happen because the first encounter
with afs_alloc_anon_key() will usually be from (auto)mount, for which all
subsequent operations must wait - apart from other (auto)mounts. Once the
call->anonymous_key is allocated, all further calls to afs_request_key()
will skip the call to afs_alloc_anon_key() for that cell.

Fixes: d27c71257825 ("afs: Fix delayed allocation of a cell's anonymous key")
Reported-by: Paulo Alcantra <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Paulo Alcantara <[e-mail]>
cc: Marc Dionne <[e-mail]>
cc: [e-mail]
cc: [e-mail]
cc: [e-mail]
Signed-off-by: Linus Torvalds <[e-mail]>

fs/afs/security.c

···
 	struct key *key;
 
 	mutex_lock(&afs_key_lock);
-	if (!cell->anonymous_key) {
+	key = cell->anonymous_key;
+	if (!key) {
 		key = rxrpc_get_null_key(cell->key_desc);
 		if (!IS_ERR(key))
 			cell->anonymous_key = key;