LU-8840 osp: handle EA cache properly (2)

LU-8840 osp: handle EA cache properly (2)

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: I85d7c7a2cafd50334f2ea0634f5e2b21c0b3908e
Reviewed-on: https://review.whamcloud.com/25207
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>