xfs: simplify xfs_calc_dquots_per_chunk

ndquots is a 32-bit value, and we don't care
about the remainder; there is no reason to use do_div
here, it seems to be the result of a decade+ historical
accident.

Worse, the do_div implementation in userspace breaks
when fed a 32-bit dividend, so we commented it out there
in any case.

Change to simple division, and then we can change
userspace to match, and mandate a 64-bit dividend in
the do_div() in userspace as well.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>