client: clean up error checking and return of _lookup_parent

client: clean up error checking and return of _lookup_parent

ll_lookup_inode can end up getting back 0 from _lookup_parent,
without zeroing out the parent pointer, which ends up remaining
uninitialized.

Fix this by moving most of the sanity checks in _lookup_parent
into ll_lookup_inode, and only have it issue the call to the MDS.

This also allows us to do the checks in a more sane order.

Fixes: http://tracker.ceph.com/issues/40085
Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit 3ade7c046c6c5eaf20517d0713a805a681128831)

Conflicts:
	src/client/Client.cc : std::lock_guard() => Mutex::Locker()
			       ceph_assert() => assert()
			       dentries.empty() => dn_set.empty()