p2p: don't discard reason set by Disconnect (#16559)

Peer.run was discarding the reason for disconnection sent to the disc
channel by Disconnect.

p2p/peer.go

@@ -220,6 +220,7 @@ loop:
 			reason = discReasonForError(err)
 			break loop
 		case err = <-p.disc:
+			reason = discReasonForError(err)
 			break loop
 		}
 	}