afs: Fix key refcounting in file locking code

[ Upstream commit 59d49076ae3e6912e6d7df2fd68e2337f3d02036 ]

Fix the refcounting of the authentication keys in the file locking code.
The vnode->lock_key member points to a key on which it expects to be
holding a ref, but it isn't always given an extra ref, however.

Fixes: 0fafdc9f ("afs: Fix file locking")
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>