[NET]: Make second arg to skb_reserved() signed.

Some subsystems, such as PPP, can send negative values
here.  It just happened to work correctly on 32-bit with
an unsigned value, but on 64-bit this explodes.

Figured out by Paul Mackerras based upon several PPP crash
reports.
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -926,7 +926,7 @@ static inline int skb_tailroom(const struct sk_buff *skb)
  *	Increase the headroom of an empty &sk_buff by reducing the tail
  *	room. This is only allowed for an empty buffer.
  */
-static inline void skb_reserve(struct sk_buff *skb, unsigned int len)
+static inline void skb_reserve(struct sk_buff *skb, int len)
 {
 	skb->data += len;
 	skb->tail += len;