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

[jhb]

Summary

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().

[dougm]

I have no objections, but it was kib, I think, who asked for the invariants in the first place, so get his ok too.

[kib]

sys/systm.h is of course very heavy weight, perhaps too heavy.

sys/vm/vm_extern.h

157–158

), (

[imp]

modulo kib's formatting nit.

[alc]

Hmm. At first this didn't make sense to me. The "assertions" were written this way so that systm.h wouldn't have to be included, but the prototype for panic() is provided by systm.h. So, this code might as well have used KASSERT all along.

[jhb]

In D34070#770387, @kib wrote:

sys/systm.h is of course very heavy weight, perhaps too heavy.

It is, but it is already a nested include in <vm/vm_page.h>.

I would not mind having a <sys/_assert.h> or the like that contained the bits need for KASSERT and friends (MPASS*, panic() prototype, etc.).