Support reconstruction of the finobt in phase 5 of xfs_repair. We
create a new cursor for the finobt and write the in-core records
that contain free inodes to the tree. Finally, pass the cursor
along to build_agi() to include the finobt root and level count in
the agi header.
Signed-off-by: Brian Foster <bfoster@redhat.com> Reviewed-by: Dave Chinner <dchinner@redhat.com> Signed-off-by: Dave Chinner <david@fromorbit.com>