Confuse about runtime.AddCleanup

[xie cui]

in this cl https://go-review.googlesource.com/c/go/+/638557/12/src/net/fd_posix.go

In my understand, fd and pfd is pointer to same slot cell in gc, why runtime.AddCleanup can not accept runtime.AddCleanup(fd, func(fd){}, fd) as cleanup arg, in my mind, I think they are fd and pfd is same to gc collector.