Make the linuxkpi's alloc_pages() consistently return wired pages.

Previously it did this only on platforms without a direct map. This
also more closely matches Linux's semantics.

Since some DRM v5.0 code assumes the old behaviour, use a
LINUXKPI_VERSION guard to preserve that until the out-of-tree module
is updated.

Reviewed by: hselasky, kib (earlier versions), johalun
MFC after: 1 week
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D20502