linuxkpi: Handle direct-mapped addresses in linux_free_kmem()

See the analysis in PR 271333. It is possible for driver code to
allocate a page, store its address as returned by page_address(), then
call free_page() on that address. On most systems that'll result in the
LinuxKPI calling kmem_free() with a direct-mapped address, which is not
legal.

Fix the problem by making linux_free_kmem() check the address to see
whether it's direct-mapped or not, and handling it appropriately.

PR: 271333, 274515
Reviewed by: hselasky, bz
Tested by: trasz
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D40028