Revert "af_unix: Call scm_recv() only after scm_set_cred()."

This reverts commit 3f5f118bb657f94641ea383c7c1b8c09a5d46ea2.

Konrad reported that desktop environment below cannot be reached after
commit 3f5f118bb657 ("af_unix: Call scm_recv() only after scm_set_cred().")

- postmarketOS (Alpine Linux w/ musl 1.2.4)
- busybox 1.36.1
- GNOME 44.1
- networkmanager 1.42.6
- openrc 0.47

Regarding to the warning of SO_PASSPIDFD, I'll post another patch to
suppress it by skipping SCM_PIDFD if scm->pid == NULL in scm_pidfd_recv().

Reported-by: Konrad Dybcio <[e-mail]>
Link: https://lore.kernel.org/netdev/[e-mail]/
Signed-off-by: Kuniyuki Iwashima <[e-mail]>
Tested-by: Ido Schimmel <[e-mail]>
Tested-by: Gal Pressman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/unix/af_unix.c

···
 	} while (size);
 
 	mutex_unlock(&u->iolock);
-	if (state->msg && check_creds)
+	if (state->msg)
 		scm_recv(sock, state->msg, &scm, flags);
 	else
 		scm_destroy(&scm);