net: fix a memleak when uncloning an skb dst and its metadata

When uncloning an skb dst and its associated metadata, a new
dst+metadata is allocated and later replaces the old one in the skb.
This is helpful to have a non-shared dst+metadata attached to a specific
skb.

The issue is the uncloned dst+metadata is initialized with a refcount of
1, which is increased to 2 before attaching it to the skb. When
tun_dst_unclone returns, the dst+metadata is only referenced from a
single place (the skb) while its refcount is 2. Its refcount will never
drop to 0 (when the skb is consumed), leading to a memory leak.

Fix this by removing the call to dst_hold in tun_dst_unclone, as the
dst+metadata refcount is already 1.

Fixes: fc4099f1 ("openvswitch: Fix egress tunnel info.")
Cc: Pravin B Shelar <pshelar@ovn.org>
Reported-by: NVlad Buslov <vladbu@nvidia.com>
Tested-by: NVlad Buslov <vladbu@nvidia.com>
Signed-off-by: NAntoine Tenart <atenart@kernel.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/dst_metadata.h

@@ -137,7 +137,6 @@ static inline struct metadata_dst *tun_dst_unclone(struct sk_buff *skb)
 #endif
 
 	skb_dst_drop(skb);
-	dst_hold(&new_md->dst);
 	skb_dst_set(skb, &new_md->dst);
 	return new_md;
 }