LinuxKPI: Implement ksize() function.

In Linux, ksize() gets the actual amount of memory allocated for a given
object. This commit adds malloc_usable_size() to FreeBSD KPI which does
the same. It also maps LinuxKPI ksize() to newly created function.

ksize() function is used by drm-kmod.

Reviewed by: hselasky, kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D26215