mm: Avoid complex heap growth math in hot path

mm: Avoid complex heap growth math in hot path

We do a lot of math about heap growth in hot path of grub_memalign().
However, the result is only used if out of memory is encountered, which
is seldom.

This patch moves these calculations away from hot path. These
calculations are now only done if out of memory is encountered. This
change can also help compiler to optimize integer overflow checks away.

Signed-off-by: Zhang Boyang <zhangboyang.id@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>