riscv pmap: add some pv list assertions

Ensure that we don't end up with a superpage in the vm_page_t's pv list.

This may help with debugging the panic reported in PR 250866, in which
l3 in pmap_remove_write() was found to be NULL. Adding a KASSERT to this
function will help narrow down the cause of this panic the next time it
occurs.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D28109