[$] Improving iov_iter

The iov_iter interface is used todescribe and iterate through buffers in the kernel. David Howells led a combined storage andfilesystem session at the 2025 Linux Storage,Filesystem, Memory Management, and BPF Summit (LSFMM+BPF) to discuss waysto improve iov_iter. His topicproposal listed a few different ideas including replacing someiov_iter types and possibly allowing mixed types in chains of iov_iterentries; he would like to make the interface itself and the uses of iov_iter inthe kernel better.