LU-6299 osp: zero-length EA xattr_set for striped_dir

LU-6299 osp: zero-length EA xattr_set for striped_dir

OSP/OUT should not assume xattr values always have
non-zero length, and OSP should be able to pack empty
xattr updates and sends to OUT.

Add test case in sanity 102r to verify empty value
xattr_set for striped dir.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I14e0172038ef4c5671ec470228edd7cc4533a89c
Reviewed-on: http://review.whamcloud.com/14787
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>