mds: lookup snapped inodes by vinodeno_t

mds: lookup snapped inodes by vinodeno_t

Directory inodes are always multiversion, snapped metadata are stored
together with head inode. lookup snapped directory inodes is basically
the same as lookup head inodes.

For non-directory inode, there can be multiple version of snapped inodes,
and they can be stored in different dirfrags. To lookup a snapped inode,
client need to tell mds from which dirfrag it got the snapped inode.

Fixes: http://tracker.ceph.com/issues/22105
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>