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>