xsk: Remove internal DMA headers

Christoph Hellwig correctly pointed out [1] that the AF_XDP core was
pointlessly including internal headers. Let us remove those includes.

  [1] https://lore.kernel.org/bpf/20201005084341.GA3224@infradead.org/



Fixes: 1c1efc2a ("xsk: Create and free buffer pool independently from umem")
Reported-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/bpf/20201005090525.116689-1-bjorn.topel@gmail.com