os.copy_file_range: save a syscall for most operations

[motiejus]

Currenty copy_file_range always uses at least two syscalls:

1. As many as it needs to do the initial copy (always 1 during my
   testing)
2. The last one is always when offset is the size of the file.

The second syscall is used to detect the terminating condition. However,
because we do a stat for other reasons, we know the size of the file,
and we can skip the syscall.

Sparse files: since copy_file_range expands holes of sparse files, I
conclude that this layer was not intended to work with sparse files. In
other words, this commit does not make it worse for sparse file society.

Test program

const std = @import("std");

pub fn main() !void {
    const arg1 = std.mem.span(std.os.argv[1]);
    const arg2 = std.mem.span(std.os.argv[2]);
    try std.fs.cwd().copyFile(arg1, std.fs.cwd(), arg2, .{});
}

Test output (current master)

Observe two copy_file_range syscalls: one with 209 bytes, one with
zero:

$ zig build-exe cp.zig
$ strace ./cp ./cp.zig ./cp2.zig |& grep copy_file_range
copy_file_range(3, [0], 5, [0], 4294967295, 0) = 209
copy_file_range(3, [209], 5, [209], 4294967295, 0) = 0
$

Test output (this diff)

Observe a single copy_file_range syscall with 209 bytes:

$ /code/zig/build/zig build-exe cp.zig
$ strace ./cp ./cp.zig ./cp2.zig |& grep copy_file_range
copy_file_range(3, [0], 5, [0], 4294967295, 0) = 209
$

[andrewrk]

Thanks for your patience on this review. With this one thing addressed, this can be merged-