xfs: create quota preallocation watermarks for realtime quota

Refactor the quota preallocation watermarking code so that it'll work
for realtime quota too.  Convert the do_div calls into div_u64 for
compactness.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>