LinuxKPI: always use contig allocations in linux_alloc_kmem()

[bz]

Summary

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

[bz]

@jhb any comments?

[dumbbell]

I just built a kernel with this patch and will use it starting from tomorrow.

[jhb]

The documentation is not entirely clear, but both core-api/dma-api-howto.rst and kernel-hacking/hacking.rst do imply that these functions allocate contiguous memory. Also, the PMAP_HAS_DMAP version in the caller of this function is careful to allocate contiguous memory always.

[dumbbell]

After three days of using it, I didn’t see any regressions.