netfilter: nf_conntrack: use RCU safe kfree for conntrack extensions

Commit 68b80f11 (netfilter: nf_nat: fix RCU races) introduced
RCU protection for freeing extension data when reallocation
moves them to a new location. We need the same protection when
freeing them in nf_ct_ext_free() in order to prevent a
use-after-free by other threads referencing a NAT extension data
via bysource list.
Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/netfilter/nf_conntrack_extend.h

@@ -86,7 +86,7 @@ static inline void nf_ct_ext_destroy(struct nf_conn *ct)
 static inline void nf_ct_ext_free(struct nf_conn *ct)
 {
 	if (ct->ext)
-		kfree(ct->ext);
+		kfree_rcu(ct->ext, rcu);
 }
 
 /* Add this type, returns pointer to data or NULL. */