Auto merge of #108283 - the8472:remove-splice-into-pipe, r=ChrisDenton

don't splice from files into pipes in io::copy

This fixes potential data ordering issue where a write performed after a copy operation could become visible in the copy even though it signaled completion.

I assumed that by not setting `SPLICE_F_MOVE` we would be safe and the kernel would do a copy in kernel space and we could avoid the read-write syscall and copy-to/from-userspace costs. But apparently that flag only makes a difference when splicing from a pipe, but not when splicing into it.

Context: https://lkml.org/lkml/2023/2/9/673

Author: bors