storage/posix: Lower log severity for ENODATA errors in getxattr()

Change-Id: I101e329cce4c1305615c63ebcb42355f6c3e85e0
BUG: 918052
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/6084
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>