xfrm: use "unsigned int" in __xfrm6_pref_hash()

x86_64 is zero-extending arch so "unsigned int" is preferred over "int"
for address calculations.

Space savings:

	add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-58 (-58)
	function                                     old     new   delta
	xfrm_hash_resize                            2752    2743      -9
	policy_hash_bysel                            985     973     -12
	policy_hash_direct                          1036     999     -37
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

net/xfrm/xfrm_hash.h

@@ -54,8 +54,8 @@ static inline unsigned int __xfrm4_dpref_spref_hash(const xfrm_address_t *daddr,
 static inline unsigned int __xfrm6_pref_hash(const xfrm_address_t *addr,
 					     __u8 prefixlen)
 {
-	int pdw;
-	int pbi;
+	unsigned int pdw;
+	unsigned int pbi;
 	u32 initval = 0;
 
 	pdw = prefixlen >> 5;     /* num of whole u32 in prefix */