gfs2: getlabel support

Add support for the GETFSLABEL ioctl in gfs2.
I tested this patch and it works as expected.

Signed-off-by: Steve Whitehouse <swhiteho@redhat.com>
Tested-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>