[$] A more dynamic software I/O TLB

The kernel's software I/O translation lookaside buffer ('swiotlb') is an
obscure corner of the DMA-support layer. The swiotlb was initially
introduced to enable DMA for devices with special challenges, and one might
have expected it to fade away as newer peripherals came along. Instead,
though, the swiotlb has turned out to be useful in places outside of its
original use cases. This
patch set from Petr Tesarik now aims to update the swiotlb with an eye
toward its continuing use indefinitely into the future.