Originally-From: Dave Chinner <dchinner@redhat.com>
Now we can read and write rmap btree blocks, we can add support to
the growfs code to initialise new rmap btree blocks.
[darrick.wong@oracle.com: fill out the rmap offset fields]
Signed-off-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>