[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_tunnel.c b/net/ipv6/ip6_tunnel.c
index 2320cc2..5383b33 100644
--- a/net/ipv6/ip6_tunnel.c
+++ b/net/ipv6/ip6_tunnel.c
@@ -838,7 +838,7 @@
 	struct dst_entry *dst;
 	struct net_device *tdev;
 	int mtu;
-	int max_headroom = sizeof(struct ipv6hdr);
+	unsigned int max_headroom = sizeof(struct ipv6hdr);
 	u8 proto;
 	int err = -1;
 	int pkt_len;