[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/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 13565df..653fc0a 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -171,7 +171,7 @@
 	u32 mtu;
 
 	if (opt) {
-		int head_room;
+		unsigned int head_room;
 
 		/* First: exthdrs may take lots of space (~8K for now)
 		   MAX_HEADER is not enough.