LU-8840 osp: handle EA cache properly

LU-8840 osp: handle EA cache properly

For success case, dt_xattr_get() should return the EA size
instead of zero. If such EA does not exist, return -ENODATA.

More code cleanup for OSP EA cache to avoid potential reference
leak, buffer overflow, and so on.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I352b99b1ed08f1b15bdb8da2bf28689ae2d61c23
Reviewed-on: https://review.whamcloud.com/23782
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>