riscv/pmap: Handle superpages in pmap_extract_and_hold()

Without this change, vm_fault_quick_hold_pages() falls back to the slow
fault handler when it encounters a superpage mapping.

Reviewed by: alc, kib
Reported and tested by: br
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D54022

(cherry picked from commit 20a0a2f8f36f5db704b674106fa61d8d95b6b071)