xfrm: use "unsigned int" in addr_match()

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

Space savings:

	add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-24 (-24)
	function                                     old     new   delta
	xfrm_state_walk                              708     696     -12
	xfrm_selector_match                          918     906     -12

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>