ceph: fix potential use-after-free in ceph_mdsc_build_path

temp is not defined outside of the RCU critical section here. Ensure
we grab that value before we drop the rcu_read_lock.

Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>