LU-3669 mdt: getxattr intent should check object existence

Make getxattr intent use HABEO_CORPUS similar to
REINT_GETXATTR to avoid crashes on race conditions.

Signed-off-by: Andrew Perepechko <andrew_perepechko@xyratex.com>
Change-Id: Ic277525152847118fbe95fcc73448945acba699d
Reviewed-on: http://review.whamcloud.com/7191
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>