LU-11107 mdt: handle nonexistent xattrs correctly

LU-11107 mdt: handle nonexistent xattrs correctly

In mdt_getxattr_pack_reply() propagate -ENODATA returns from
mo_xattr_list() to mdt_getxattr(). Add sanity test_102s() to ensure
that getting a nonexistint xattr will fail.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: Ic7a01feb3fcac66d39f84b4ebdfc86025c3e2779
Reviewed-on: https://review.whamcloud.com/32753
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>