A change in maintenance for the kernel's DMA-mapping layer

The conversation around the merging of a set of Rust abstractions for the
kernel's DMA-mapping layer has mostly settled after Linus Torvalds made it clear that the code would be
accepted. One other consequence of this decision, though, is that
Christoph Hellwig has quietly stepped down from the
maintenance of the DMA-mapping code. Marek Szyprowski will be the
maintainer of that layer going forward. Hellwig has maintained that code
for many years; his contributions will be missed.