dma-contiguous: fix !CONFIG_DMA_CMA version of dma_{alloc, free}_contiguous()

Commit fdaeec198ada ("dma-contiguous: add dma_{alloc,free}_contiguous()
helpers") adds a pair of new helper functions so as to abstract code in
the dma-direct (and other places in the future), however it breaks QEMU
boot feature using x86_64 defconfig.

That's because x86_64 defconfig has CONFIG_DMA_CMA=n so those two newly
introduced helper functions are empty in their !CONFIG_DMA_CMA version,
while previously the platform independent dma-direct code had fallback
alloc_pages_node() and __free_pages().

So this patch fixes it by adding alloc_pages_node() and __free_pages()
in the !CONFIG_DMA_CMA version of the two helper functions.

Tested with below QEMU command:
  qemu-system-x86_64 -m 512m \
      -drive file=images/x86_64/rootfs.ext4,format=raw,if=ide \
      -append 'console=ttyS0 root=/dev/sda' -nographic \
      -kernel arch/x86_64/boot/bzImage

with the rootfs from the below link:
  https://github.com/ClangBuiltLinux/continuous-integration/raw/master/images/x86_64/rootfs.ext4



Fixes: fdaeec198ada ("dma-contiguous: add dma_{alloc,free}_contiguous() helpers")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>