GFS2: Add UUID to GFS2 sb

This patch adds a UUID to the GFS2 sb structure. This field is not
actually referenced from kernel space at all, but is added for
completeness and due to the userland tools which get their on-disk
structure information from the gfs2_ondisk.h header file.

Since we have to be backwards compatible, we will assume that any GFS2
sb for which the UUID is all 0 does not have a UUID as such.

We should then be (after some userland changes) able to support the -U
mount option. This addresses Fedora bugzilla #242689

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