client: add ceph_ll_sync_inode

client: add ceph_ll_sync_inode

Ganesha's NFS SETATTR requests are expected to be synchronous, but
libcephfs can cache attribute changes if the client has the right caps.
An fsync after changing the attributes would flush the caps back to the
MDS, but we may not have an Fh in the context of a SETATTR request.

Add a way to sync a specific Inode, even when we don't have easy access
to a Fh.

Tracker: http://tracker.ceph.com/issues/23291
Signed-off-by: Jeff Layton <jlayton@redhat.com>