ceph_d_revalidate(): use stable parent inode passed by caller

ceph_d_revalidate(): use stable parent inode passed by caller

No need to mess with the boilerplate for obtaining what we already
have.  Note that ceph is one of the "will want a path from filesystem
root if we want to talk to server" cases, so the name of the last
component is of little use - it is passed to fscrypt_d_revalidate()
and it's used to deal with (also crypt-related) case in request
marshalling, when encrypted name turns out to be too long.  The former
is not a problem, but the latter is racy; that part will be handled
in the next commit.

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