client: Add lookup_parent & extend lookup_ino

lookup_parent enables CEPH_MDS_OP_LOOKUPPARENT
operation.

Extend lookup_ino to 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 the lookup_ino and
lookup_parent functions in Client, 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>