internal/poll: can update newPipe to take advantage of newer Linux kernel requirement

[dmitshur]

newPipe currently contains this comment:

// pipe2 was added in 2.6.27 and our minimum requirement is 2.6.23, so it
// might not be implemented. Falling back to pipe is possible, but prior to
// 2.6.29 splice returns -EAGAIN instead of 0 when the connection is
// closed.

(src/internal/poll/splice_linux.go#220-223)

It was added in 2018 to fix #25486.

As of proposal #45964, the minimum kernel that needs to be supported is 2.6.32, so there may be an opportunity to take advantage of that. If there's nothing to do differently, the comment can be deleted since pipe2 will now always be available and there's no need to explain why to avoid fallback to pipe.

CC @ianlancetaylor, @bradfitz, @tklauser.

[gopherbot]

Change https://go.dev/cl/422979 mentions this issue: internal/poll: remove detection of buggy splice on old Linux versions