split receive_fd_replace from __receive_fd
From: Christoph Hellwig
Date: Thu Mar 25 2021 - 04:23:21 EST
The receive_fd_replace case shares almost no logic with the more general
__receive_fd case, so split it into a separate function.
BTW, I'm not sure if receive_fd_replace is such a useful primitive to
start with, why not just open code it in seccomp?