mds: save the oldest snapid of inode

mds: save the oldest snapid of inode

When moving a inode to different snapshot realm, we need to check if
the inode is snapshotted. If the inode is snapshotted, we create a
snapshot realm for it. The snapshot realm records inode's past snapshot
realm.

Currently there is no reliable way to check if a inode is snapshotted.
So add a new field to CInode the store the oldest snapid, it's used to
check if inode is snapshotted.

Signed-off-by: Yan, Zheng <zyan@redhat.com>