mds: Return EINVAL for parent lookup on root ino

This was the behaviour before 8d6b25a1, and distinguishes
the general failure to find a parent (ESTALE) from the specific
case of a node being present but being the base (EINVAL)

Signed-off-by: John Spray <john.spray@inktank.com>
client: Add lookup_parent & lookup_name

Exposing the CEPH_MDS_OP_LOOKUPPARENT and
recently added CEPH_MDS_OP_LOOKUPNAME operations.

lookup_ino and lookup_parent optionally take an Inode**
which will be populated with a reference to the
resulting inode.

Fixes: #3863

Signed-off-by: John Spray <john.spray@inktank.com>
libcephfs: add ceph_ll_lookup_inode

This is a wrapper to lookup_ino, lookup_parent
and lookup_name, to provide the
caller with the requested inode and its
dentry.  The result is suitable for traversing
the inode's path.

Fixes: #3863

Signed-off-by: John Spray <john.spray@inktank.com>
tools: Create ceph-client-debug

For exposing test/dev functionality which uses
the cephfs client interface.

Initially just provides lookup-by-ino.

Fixes: #3863

Signed-off-by: John Spray <john.spray@inktank.com>