staging/lustre/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.

Signed-off-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-on: http://review.whamcloud.com/12952
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-3544
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>