LinuxKPI: Add kmalloc_size_roundup function

kmalloc_size_roundup reports allocation bucket size for the given size.

Sponsored by: Serenity CyberSecurity, LLC
MFC after: 1 week
Reviewed by: manu, emaste
Differential Revision: https://reviews.freebsd.org/D45841

(cherry picked from commit ab1a5d8e2ade8c706ff8cd7aa3d5c88a95633228)