[NET]: Conditionally use bh_lock_sock_nested in sk_receive_skb

Spotted by Ian McDonald, tentatively fixed by Gerrit Renker:

http://www.mail-archive.com/dccp%40vger.kernel.org/msg00599.html

Rewritten not to unroll sk_receive_skb, in the common case, i.e. no lock
debugging, its optimized away.

Signed-off-by: Arnaldo Carvalho de Melo <acme@mandriva.com>

net/decnet/dn_nsp_in.c

@@ -804,7 +804,7 @@ got_it:
 				goto free_out;
 		}
 
-		return sk_receive_skb(sk, skb);
+		return sk_receive_skb(sk, skb, 0);
 	}
 
 	return dn_nsp_no_socket(skb, reason);