GFS2: Use UUID field in generic superblock

The VFS superblock structure now has a UUID field, so we can use that
in preference to the UUID field in the GFS2 superblock now.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

diff --cc fs/gfs2/incore.h
Simple merge

diff --cc fs/gfs2/ops_fstype.c
Simple merge

diff --cc fs/gfs2/sys.c
Simple merge