net: ping6: Fix ping -6 with interface name

When passing interface parameter to ping -6:
$ ping -6 ::11:141:84:9 -I eth2
Results in:
PING ::11:141:84:10(::11:141:84:10) from ::11:141:84:9 eth2: 56 data bytes
ping: sendmsg: Invalid argument
ping: sendmsg: Invalid argument

Initialize the fl6's outgoing interface (OIF) before triggering
ip6_datagram_send_ctl. Don't wipe fl6 after ip6_datagram_send_ctl() as
changes in fl6 that may happen in the function are overwritten explicitly.
Update comment accordingly.

Fixes: 13651224 ("net: ping6: support setting basic SOL_IPV6 options via cmsg")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220531084544.15126-1-tariqt@nvidia.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

net/ipv6/ping.c

@@ -101,6 +101,9 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	ipc6.sockc.tsflags = sk->sk_tsflags;
 	ipc6.sockc.mark = sk->sk_mark;
 
+	memset(&fl6, 0, sizeof(fl6));
+	fl6.flowi6_oif = oif;
+
 	if (msg->msg_controllen) {
 		struct ipv6_txoptions opt = {};
 
@@ -112,17 +115,14 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 			return err;
 
 		/* Changes to txoptions and flow info are not implemented, yet.
-		 * Drop the options, fl6 is wiped below.
+		 * Drop the options.
 		 */
 		ipc6.opt = NULL;
 	}
 
-	memset(&fl6, 0, sizeof(fl6));
-
 	fl6.flowi6_proto = IPPROTO_ICMPV6;
 	fl6.saddr = np->saddr;
 	fl6.daddr = *daddr;
-	fl6.flowi6_oif = oif;
 	fl6.flowi6_mark = ipc6.sockc.mark;
 	fl6.flowi6_uid = sk->sk_uid;
 	fl6.fl6_icmp_type = user_icmph.icmp6_type;