riscv/pmap: Handle superpages in pmap_extract_and_hold()

[markj]

Summary

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

[markj]

Return failure if the L2 entry is invalid

[markj]

Fix handling of insufficient permissions.

[alc]

sys/riscv/riscv/pmap.c

1142–1143

The l3p != NULL is now redundant.

1143–1155

This goto is redundant.

[markj]

Handle Alan's comments.

[kib]

sys/riscv/riscv/pmap.c

1159

Then this part could become just under 'else' and then goto is not needed at all.
But the current version is fine as well.

[markj]

Avoid gotos

[kib]

Thanks.