ipv6: fix restrict IPV6_ADDRFORM operation

BugLink: https://bugs.launchpad.net/bugs/1878098

[ Upstream commit 82c9ae44 ]

Commit b6f61189 ("ipv6: restrict IPV6_ADDRFORM operation") fixed a
problem found by syzbot an unfortunate logic error meant that it
also broke IPV6_ADDRFORM.

Rearrange the checks so that the earlier test is just one of the series
of checks made before moving the socket from IPv6 to IPv4.

Fixes: b6f61189 ("ipv6: restrict IPV6_ADDRFORM operation")
Signed-off-by: John Haxby <john.haxby@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ian May <ian.may@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

net/ipv6/ipv6_sockglue.c

@@ -185,15 +185,14 @@ static int do_ipv6_setsockopt(struct sock *sk, int level, int optname,
 					retv = -EBUSY;
 					break;
 				}
-			} else if (sk->sk_protocol == IPPROTO_TCP) {
-				if (sk->sk_prot != &tcpv6_prot) {
-					retv = -EBUSY;
-					break;
-				}
-				break;
-			} else {
+			}
+			if (sk->sk_protocol == IPPROTO_TCP &&
+			    sk->sk_prot != &tcpv6_prot) {
+				retv = -EBUSY;
 				break;
 			}
+			if (sk->sk_protocol != IPPROTO_TCP)
+				break;
 			if (sk->sk_state != TCP_ESTABLISHED) {
 				retv = -ENOTCONN;
 				break;