pmap_growkernel: Use VM_ALLOC_NOFREE when allocating pagetable pages

[bnovkov]

Summary

This patch modifies pmap_growkernel in all pmaps to use VM_ALLOC_NOFREE when allocating new pagetable pages.
This should help reduce longterm fragmentation as these pages are never released after they are allocated.

Test Plan

Tested with the regression suite for all architectures except powerpc.

[mhorne]

Nice. The flag acts as a meaningful annotation, regardless of any effects on fragmentation.

[bnovkov]

Closed by ddc09a1.