LinuxKPI: always use contig allocations in linux_alloc_kmem()

In linux_alloc_kmem() [used by *get_page*()] we always at least allocate
PAGE_SIZE and we want the allocation to be contiguous so it can be passed
to DMA. Always use kmem_alloc_contig() and only change the low argument
depending on the GFP_DMA32 flag being given or not.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: jhb, dumbbell
Differential Revision: https://reviews.freebsd.org/D46661