ceph: fix cred leak in ceph_mds_check_access()

get_current_cred() increments the reference counter, but the
put_cred() call was missing.

Cc: stable@vger.kernel.org
Fixes: 596afb0b ("ceph: add ceph_mds_check_access() helper")
Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>