Make the df calculations of available space the same as that done in

Make the df calculations of available space the same as that done in
ffs_statfs().

In 1998, with /usr/src/sys/ufs/ffs/ffs_vfsops.c r1.16, mickey@
improved the calculation of available blocks in ffs_statfs().

Yesterday guenther@ noticed that this fix had not been applied to the
same calculation being done by df(8) when run against raw
devices. Which meant different values were displayed depending on
which device/file system you gave to df.

ok otto@