xfs: add finobt support to growfs

Add finobt support to growfs. Initialize the agi root/level fields
and the root finobt block.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>