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.

Test-Parameters: alwaysuploadlogs envdefinitions=SLOW=yes testlist=parallel-scale-nfs,parallel-scale-nfs
Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Change-Id: Id6fbbef339fea540728b181b2ae91799b8151fbd
Reviewed-on: http://review.whamcloud.com/10693
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>