mds: use cap on head if there is none on the snapped inode
mds: use cap on head if there is none on the snapped inode

This is needed, in particular, when we're flushing snap data on an inode
that already got COWed.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph