ceph: clear d_fsinfo pointer under d_lock

To check for a valid dentry lease, we need to get at the
ceph_dentry_info. Under rcuwalk though, we may end up with a dentry that
is on its way to destruction. Since we need to take the d_lock in
dentry_lease_is_valid already, we can just ensure that we clear the
d_fsinfo pointer out under the same lock before destroying it.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>