Fix some issues with the riscv pmap_protect() implementation.

• Handle VM_PROT_EXECUTE.
• Clear PTE_D and mark the page dirty when removing write access from a mapping.
• Atomically clear PTE_W to avoid clobbering a hardware PTE update.

Reviewed by: jhb, kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18719