Add back io::copy file to pipe optimization

So to sum up, I think we can add back optimization via:

• if the file is on squashfs/erofs, then it's immutable (detectable via statfs
• if the is sealed with F_SEAL_WRITE (prevent modifying of existing contents, shrinking and appending is allowed) and F_SEAL_SEAL (not changing of seals), detectable via fcntl(fd, F_GET_SEALS)
• if it's on read-only btrfs subvolume (e.g. snapshot), likewise we can do the same for zfs, nilfs, bcachefs
• If the file is /dev/zero, or from other pseudo fs (e.g. procfs, sysfe), then I think we can assume it's immutable

Otherwise, we'd fallback to:

• read + write if the file is really small
• splice to an intermediate pipe with F_MOVE and then splice to the pipe without F_MOVE if the file is more than 1-2 pages
• if the fs supports reflink and file is really large, then we could create a O_TMPFILE on the system, copy_file_range to it and then splice it

Also, if the file is a named fifo, then just a splice without F_MOVE would work fine.

Originally posted by @NobodyXu in https://github.com/rust-lang/rust/issues/108283#issuecomment-2254439139

Start by tracing the io::copy implementation and reading the referenced statfs, fcntl, splice, and copy_file_range APIs. Compare the proposed filesystem and file-type cases with the existing behavior; done means restoring the optimization while preserving the listed fallbacks and handling named FIFOs correctly.