net/tls: fix refcount adjustment in fallback

Unlike atomic_add(), refcount_add() does not deal well
with a negative argument.  TLS fallback code reallocates
the skb and is very likely to shrink the truesize, leading to:

[  189.513254] WARNING: CPU: 5 PID: 0 at lib/refcount.c:81 refcount_add_not_zero_checked+0x15c/0x180
 Call Trace:
  refcount_add_checked+0x6/0x40
  tls_enc_skb+0xb93/0x13e0 [tls]

Once wmem_allocated count saturates the application can no longer
send data on the socket.  This is similar to Eric's fixes for GSO,
TCP:
commit 7ec318fe ("tcp: gso: avoid refcount_t warning from tcp_gso_segment()")
and UDP:
commit 575b65bc ("udp: avoid refcount_t saturation in __udp_gso_segment()").

Unlike the GSO case, for TLS fallback it's likely that the skb has
shrunk, so the "likely" annotation is the other way around (likely
branch being "sub").

Fixes: e8f69799 ("net/tls: Add generic NIC offload infrastructure")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_device_fallback.c

@@ -194,6 +194,9 @@ static void update_chksum(struct sk_buff *skb, int headln)
 
 static void complete_skb(struct sk_buff *nskb, struct sk_buff *skb, int headln)
 {
+	struct sock *sk = skb->sk;
+	int delta;
+
 	skb_copy_header(nskb, skb);
 
 	skb_put(nskb, skb->len);
@@ -201,11 +204,15 @@ static void complete_skb(struct sk_buff *nskb, struct sk_buff *skb, int headln)
 	update_chksum(nskb, headln);
 
 	nskb->destructor = skb->destructor;
-	nskb->sk = skb->sk;
+	nskb->sk = sk;
 	skb->destructor = NULL;
 	skb->sk = NULL;
-	refcount_add(nskb->truesize - skb->truesize,
-		     &nskb->sk->sk_wmem_alloc);
+
+	delta = nskb->truesize - skb->truesize;
+	if (likely(delta < 0))
+		WARN_ON_ONCE(refcount_sub_and_test(-delta, &sk->sk_wmem_alloc));
+	else if (delta)
+		refcount_add(delta, &sk->sk_wmem_alloc);
 }
 
 /* This function may be called after the user socket is already