GFS2: Add get_xquota support

This adds support for viewing the current GFS2 quota settings
via the XFS quota API. The setting of quotas will be addressed
in a later patch. Fields which are not supported here are left
set to zero.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>