xfrm: Convert xfrm_addr_cmp() to boolean xfrm_addr_equal().

All users of xfrm_addr_cmp() use its result as boolean.
Introduce xfrm_addr_equal() (which is equal to !xfrm_addr_cmp())
and convert all users.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>