LinuxKPI: make __kmalloc() play by the rules

According to Documentation/core-api/dma-api.rst kmalloc() is supposd
to provide physically contiguous memory. [1]

In order to guarantee that allocations are contiguous even if using
PAGE_SIZE or larger check the size and use contigmalloc if needed.
This makes use of 9e6544dd6e02 (and following) allowing free(9) to
also work for contigmalloced memory.

Sponsored by: The FreeBSD Foundation
Pointed out by: jhb [1]
Reviewed by: jhb, emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46656