[$] A more dynamic software I/O TLB

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