unix/kernel_copy.rs: copy_file_range_candidate allows empty output files
This is for https://github.com/rust-lang/rust/issues/114341
The `meta.len() > 0` condition here is intended for inputs only, ie. when input is in the `/proc` filesystem as documented.
That inaccurately included empty output files which are then shunted to the sendfile() routine leading to higher than nescessary IO util in some cases, specifically with CoW filesystems like btrfs.
Simply, determine what is input or output given the passed boolean.
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