Avoid erroneously clearing PGA_WRITEABLE in riscv's pmap_enter().

During a CoW fault, we must check for both 4KB and 2MB mappings before
clearing PGA_WRITEABLE on the old mapping's page. Previously we were
only checking for 4KB mappings. This was missed in r344106.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation