riscv: handle kernel PTE edge-case in pmap_enter_l2()

[mhorne]

Summary

Page table pages are never freed from the kernel pmap, instead they are
zeroed when a range is unmapped. This allows future mappings to be
constructed more quickly. Detect this scenario in pmap_enter_l2(), so we
don't fail to create a superpage mapping when the 2MB range is actually
available.

[alc]

sys/riscv/riscv/pmap.c

3204–3205

On amd64 and arm64, we don't maintain an exact count of the valid mappings within a kernel page table page. If that is true on riscv, then this section of code should look more like the amd64 and arm64 versions.

[mhorne]

sys/riscv/riscv/pmap.c

3204–3205

Yes, good catch. I will make the adjustment as part of D36563.