netns: constify net_hash_mix() and various callers

const qualifiers ease code review by making clear
which objects are not written in a function.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/linux/udp.h b/include/linux/udp.h
index 247cfdc..87c0949 100644
--- a/include/linux/udp.h
+++ b/include/linux/udp.h
@@ -34,7 +34,7 @@
 
 #define UDP_HTABLE_SIZE_MIN		(CONFIG_BASE_SMALL ? 128 : 256)
 
-static inline int udp_hashfn(struct net *net, unsigned num, unsigned mask)
+static inline u32 udp_hashfn(const struct net *net, u32 num, u32 mask)
 {
 	return (num + net_hash_mix(net)) & mask;
 }