fs: Move __scm_install_fd() to __receive_fd()

In preparation for users of the "install a received file" logic outside
of net/ (pidfd and seccomp), relocate and rename __scm_install_fd() from
net/core/scm.c to __receive_fd() in fs/file.c, and provide a wrapper
named receive_fd_user(), as future patches will change the interface
to __receive_fd().

Additionally add a comment to fd_install() as a counterpoint to how
__receive_fd() interacts with fput().

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Dmitry Kadashev <dkadashev@gmail.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Sargun Dhillon <sargun@sargun.me>
Cc: Ido Schimmel <idosch@idosch.org>
Cc: Ioana Ciornei <ioana.ciornei@nxp.com>
Cc: linux-fsdevel@vger.kernel.org
Cc: netdev@vger.kernel.org
Reviewed-by: Sargun Dhillon <sargun@sargun.me>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Kees Cook <keescook@chromium.org>

net/compat.c

@@ -298,7 +298,7 @@ void scm_detach_fds_compat(struct msghdr *msg, struct scm_cookie *scm)
 	int err = 0, i;
 
 	for (i = 0; i < fdmax; i++) {
-		err = __scm_install_fd(scm->fp->fp[i], cmsg_data + i, o_flags);
+		err = receive_fd_user(scm->fp->fp[i], cmsg_data + i, o_flags);
 		if (err)
 			break;
 	}