LinuxKPI: Add more #ifdef VM_MEMATTR_WRITE_COMBINING guards

One of the three uses is already guarded; this guards the remaining ones
to support architectures like riscv that do not provide write-combining,
and is needed to build drm-kmod on riscv.

Reviewed by: hselasky, manu
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D31999