xfrm: optimize ipv4 selector matching

Current addr_match() is errh, under-optimized.

Compiler doesn't know that memcmp() branch doesn't trigger for IPv4.
Also, pass addresses by value -- they fit into register.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>