riscv: Fix pmap_protect for superpages

When protecting a superpage, we would previously fall through to the
non-superpage case and read the contents of the superpage as PTEs,
potentially modifying them and trying to look up underlying VM pages that
don't exist if they happen to look like PTEs we would care about. This led
to nginx causing an unexpected page fault in pmap_protect that panic'ed the
kernel. Instead, if we see a superpage, we are done for this range and
should continue to the next.

Reviewed by: markj, jhb (mentor)
Approved by: markj, jhb (mentor)
Differential Revision: https://reviews.freebsd.org/D24827