LU-11109 mdt: handle zero length xattr values correctly

LU-11109 mdt: handle zero length xattr values correctly

In mdt_getxattr(), set OBD_MD_FLXATTR in mbo_valid of the reply's MDT
body so that the client can distinguish between nonexistent extended
attributes and zero length values. In ll_xattr_list() and
ll_getxattr_common() test for OBD_MD_FLXATTR and return 0 rather than
-ENODATA in the appropriate cases. Add sanity test_102t() to test that
zero length values are handled correctly.

Signed-off-by: John L. Hammond <jhammond@whamcloud.com>
Change-Id: I15649581c26dc52e83ca714b44f8372f29954ed5
Reviewed-on: https://review.whamcloud.com/32755
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>