client: add ceph_ll_lookup_vino

client: add ceph_ll_lookup_vino

Add a new API function for looking up an inode via a vinodeno_t. This
should give ganesha a way to reliably look up snapshot inodes.

We do need to add some special handling for CEPH_SNAPDIRs. If we're
looking for one, then find the non-snapped parent, and then call
open_snapdir to get the snapdir inode.

Also, have the function check the local cache before calling the MDS
to look up an inode.

Fixes: https://tracker.ceph.com/issues/48991
Signed-off-by: Jeff Layton <jlayton@redhat.com>
(cherry picked from commit 70622079c2ec55222a139fa5042902e0b19bd839)

Conflicts:
	src/client/Client.cc