[GFS2] Change ondisk format (hopefully for the last time)

There were one or two fields in structures which didn't get changed
last time back to their gfs1 sizes and alignments. One or two constants
have also changed back to their original values which were missed the
first time.

Its possible that indirect pointer blocks might need to change. If
they don't we'll have to rewrite them all on upgrade due to a change
in the amount of padding that they use.

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