LU-184 Keep orphan on failover umount

LU-184 Keep orphan on failover umount

- On failover umount, the orphan should not be cleared on last close.
- In mdt_mfd_close(), the ma_valid should not be set as zero, otherwise,
  mds will never issue object destroy to OSS.

Issue:
Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: If0a894655d0ea42ad0d07e725174b73a137b7adb
Reviewed-on: http://review.whamcloud.com/426
Tested-by: Hudson
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>