storage/posix: Set errno for xattrop failures

Change-Id: I4d44068c8da5257227d62906ec18ae16f6ed6c02
BUG: 1172477
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/9261
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Tested-by: Raghavendra Bhat <raghavendra@redhat.com>