Make <vm/vm_extern.h> more self-contained.

Add a nested include of <sys/systm.h> for recently added assertions.
Without this, existing code (such as in drm-kmod) needs to be patched
to add the newly required header.

While here, rewrite the assertions using KASSERT().

Reviewed by: dougm, alc, imp, kib
Differential Revision: https://reviews.freebsd.org/D34070