ipv6: release dst in ping_v6_sendmsg

[Dave Jones]

Neither the failure or success paths of ping_v6_sendmsg release
the dst it acquires.  This leads to a flood of warnings from
"net/core/dst.c:288 dst_release" on older kernels that
don't have 8bf4ada2e21378816b28205427ee6b0e1ca4c5f1 backported.

That patch optimistically hoped this had been fixed post 3.10, but
it seems at least one case wasn't, where I've seen this triggered
a lot from machines doing unprivileged icmp sockets.

Cc: Martin Lau <ka...@fb.com>
Signed-off-by: Dave Jones <da...@codemonkey.org.uk>

diff --git a/net/ipv6/ping.c b/net/ipv6/ping.c
index 0900352c924c..0e983b694ee8 100644
--- a/net/ipv6/ping.c
+++ b/net/ipv6/ping.c
@@ -126,8 +126,10 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr 
*msg, size_t len)
        rt = (struct rt6_info *) dst;
 
        np = inet6_sk(sk);
-       if (!np)
-               return -EBADF;
+       if (!np) {
+               err = -EBADF;
+               goto dst_err_out;
+       }
 
        if (!fl6.flowi6_oif && ipv6_addr_is_multicast(&fl6.daddr))
                fl6.flowi6_oif = np->mcast_oif;
@@ -163,6 +165,9 @@ static int ping_v6_sendmsg(struct sock *sk, struct msghdr 
*msg, size_t len)
        }
        release_sock(sk);
 
+dst_err_out:
+       dst_release(dst);
+
        if (err)
                return err;