xfs: update inode allocation/free transaction reservations for finobt

Create the xfs_calc_finobt_res() helper to calculate the finobt log
reservation for inode allocation and free. Update
XFS_IALLOC_SPACE_RES() to reserve blocks for the additional finobt
insertion on inode allocation. Create XFS_IFREE_SPACE_RES() to
reserve blocks for the potential finobt record insertion on inode
free (i.e., if an inode chunk was previously fully allocated).

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