Use inline tests for individual PTE bits in the RISC-V pmap.

Inline tests for PTE_* bits are easy to read and don't really require a
predicate function, and predicates which operate on a pt_entry_t are
inconvenient when working with L1 and L2 page table entries.

Reviewed by: jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18461