net: dccp: fix sign bug

'gap' is unsigned, so this code is wrong:

    gap = -new_head;
    ...
    if (gap > 0) { ... }

Make 'gap' signed.

The semantic patch that finds this problem (many false-positive results):
(http://coccinelle.lip6.fr/)

// <smpl>
@ r1 @
identifier f;
@@
int f(...) { ... }

@@
identifier r1.f;
type T;
unsigned T x;
@@

*x = f(...)
 ...
*x > 0
Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dccp/ackvec.c

@@ -201,7 +201,7 @@ static inline int dccp_ackvec_set_buf_head_state(struct dccp_ackvec *av,
 						 const unsigned int packets,
 						 const unsigned char state)
 {
-	unsigned int gap;
+	long gap;
 	long new_head;
 
 	if (av->av_vec_len + packets > DCCP_MAX_ACKVEC_LEN)