xfs_db: disallow sb UUID write on v5 filesystems

xfs_db: disallow sb UUID write on v5 filesystems

Do not allow xfs_db (or the xfs_admin frontend) to change the UUID
of a V5 filesystem; this will cause UUID mismatches across the
filesystem, and we currently have no mechanism to update them all.
Changing only the superblock UUID makes all other metadata look
invalid, and xfs_repair reacts by junking everything.

Addresses-Debian-Bug: 782012
Reported-by: F. Stoyan <fstoyan@swapon.de>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>