swiotlb: don't dip into swiotlb pool for coherent allocations

All architectures that support swiotlb also have a zone that backs up
these less than full addressing allocations (usually ZONE_DMA32).

Because of that it is rather pointless to fall back to the global swiotlb
buffer if the normal dma direct allocation failed - the only thing this
will do is to eat up bounce buffers that would be more useful to serve
streaming mappings.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>