mds: fix race between scatter gather and dirfrag export

If we gather dirty scatter lock state while corresponding dirfrag
is been exporting, we may receive different dirfrag states from
two MDS and we need to find which one is the newest. The solution
is adding a new variable "migrate seq" to dirfrag, increase it by
one when dirfrag's auth MDS changes. When gathering dirty scatter
lock state, use "migrate seq" to find the newest dirfrag state.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>
mds: fix remote wrlock rejoin

remote wrlock's target is not always inode's auth MDS.

Signed-off-by: Yan, Zheng <zheng.z.yan@intel.com>