gfs2: fs: derive f_fsid from s_uuid

gfs2 already has optional persistent uuid.

Use that uuid to report f_fsid in statfs(2), same as ext2/ext4/zonefs.

This allows gfs2 to be monitored by fanotify filesystem watch.
for example, with inotify-tools 4.23.8.0, the following command can be
used to watch changes over entire filesystem:

  fsnotifywatch --filesystem /mnt/gfs2

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>