LU-3544 xattr: xattr data may be gone with lock held

LU-3544 xattr: xattr data may be gone with lock held

Xattr cached data may be gone, but lock still held, in this case,
refetch xattr from server, otherwise client will return error.

This patch is backported from the following one:
Lustre-commit: Id6fbbef339fea540728b181b2ae91799b8151fbd
Lustre-Change: http://review.whamcloud.com/10693

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Ide3c2686a7a9f118c4c44fc5de8eb5670011f112
Reviewed-on: http://review.whamcloud.com/12952
Tested-by: Jenkins
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>