ceph: fix a race with rename() in ceph_mdsc_build_path()

ceph: fix a race with rename() in ceph_mdsc_build_path()

Lift copying the name into callers of ceph_encode_encrypted_dname()
that do not have it already copied; ceph_encode_encrypted_fname()
disappears.

That fixes a UAF in ceph_mdsc_build_path() - while the initial copy
of plaintext into buf is done under ->d_lock, we access the
original name again in ceph_encode_encrypted_fname() and that is
done without any locking.  With ceph_encode_encrypted_dname() using
the stable copy the problem goes away.

Tested-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Reviewed-by: Viacheslav Dubeyko <Slava.Dubeyko@ibm.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>