[NET]: Treat the sign of the result of skb_headroom() consistently

In some places, the result of skb_headroom() is compared to an unsigned
integer, and in others, the result is compared to a signed integer.  Make
the comparisons consistent and correct.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index f508835..e5f7dc2 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -161,7 +161,7 @@
 	struct dst_entry *dst = skb->dst;
 	struct rtable *rt = (struct rtable *)dst;
 	struct net_device *dev = dst->dev;
-	int hh_len = LL_RESERVED_SPACE(dev);
+	unsigned int hh_len = LL_RESERVED_SPACE(dev);
 
 	if (rt->rt_type == RTN_MULTICAST)
 		IP_INC_STATS(IPSTATS_MIB_OUTMCASTPKTS);