udp: Unbreak modules that rely on external __skb_recv_udp() availability

Commit 2276f58a ("udp: use a separate rx queue for packet reception")
turned static inline __skb_recv_udp() from being a trivial helper around
__skb_recv_datagram() into a UDP specific implementaion, making it
EXPORT_SYMBOL_GPL() at the same time.

There are external modules that got broken by __skb_recv_udp() not being
visible to them. Let's unbreak them by making __skb_recv_udp EXPORT_SYMBOL().

Rationale (one of those) why this is actually "technically correct" thing
to do: __skb_recv_udp() used to be an inline wrapper around
__skb_recv_datagram(), which itself (still, and correctly so, I believe)
is EXPORT_SYMBOL().

Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Eric Dumazet <edumazet@google.com>
Fixes: 2276f58a ("udp: use a separate rx queue for packet reception")
Signed-off-by: NJiri Kosina <jkosina@suse.cz>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -1627,7 +1627,7 @@ struct sk_buff *__skb_recv_udp(struct sock *sk, unsigned int flags,
 	*err = error;
 	return NULL;
 }
-EXPORT_SYMBOL_GPL(__skb_recv_udp);
+EXPORT_SYMBOL(__skb_recv_udp);
 
 /*
  * 	This should be easy, if there is something there we