riscv: optimize MADV_WILLNEED on existing superpages

Specifically, avoid pointless calls to pmap_enter_quick_locked() when
madvise(MADV_WILLNEED) is applied to an existing superpage mapping.

1d5ebad06c20 made the change for amd64 and arm64.

Reviewed by: markj, alc
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36563