file_unix: let copy_file_range() manage the fd's offsets
The method signature is: ssize_t copy_file_range(int fd_in, off64_t *off_in, int fd_out, off64_t *off_out, size_t len, unsigned int flags);
and the docs say: If off_in is NULL, then bytes are read from fd_in starting from the file offset, and the file offset is adjusted by the number of bytes copied.
(the same goes for off_out).
So using nullptr is more robust; and we just add the copiedBytes to sizeProcessed.
[1]https://man7.org/linux/man-pages/man2/copy_file_range.2.html
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file_unix: use copy_file_range on FreeBSD too
See https://www.freebsd.org/cgi/man.cgi?query=copy_file_range&sektion=2&format=html
Edited by Ahmad Samir