afs: Fix key ref leak in afs_put_operation()

The afs_put_operation() function needs to put the reference to the key
that's authenticating the operation.

Fixes: e49c7b2f

 ("afs: Build an abstraction around an "operation" concept")
Reported-by: Dave Botsch <botsch@cnf.cornell.edu>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>