Fix next_multiple for mmap-alloc

[Dr-Emann]

The remainder should be subtracted from the unit, not the size: e.g.

next_multiple(4, 4096); // returns 4
next_multiple(10000, 4096); // returns 18192

[joshlf]

Thanks for the PR, and good catch!

A couple of small fixes :

• Can you change the commit message to be something like mmap-alloc: Fix bug in next_multiple? (see commit message guidelines)
• Can you add an entry for this in the "Fixed" section of the crate's CHANGELOG.md file?

Other than that, looks good!

[joshlf]

Thanks!