Storage/posix: Don't log at ERROR level for failed getxattr.

Problem:
ENOATTR returned by
getxattr -n <NotAnExistingAttribute> <file>
was being logged at ERROR level.

Solution:
Moved logging to DEBUG level.

Change-Id: I982a577a4c231faa958ea71abdb272f8d5ffd70c
BUG: 918052
Signed-off-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-on: http://review.gluster.org/4628
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>