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

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.

Reviewed by: markj
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36885

(cherry picked from commit 1f9cc5ffc50523498545b6f7d50406feb18903be)