selftests: fib_rule_tests: use pre-defined DEV_ADDR

DEV_ADDR is defined but not used. Use it in address setting.
Do the same with IPv6 for consistency.

Reported-by: David Ahern <dsahern@gmail.com>
Fixes: fc82d93e

 ("selftests: fib_rule_tests: fix local IPv4 address typo")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>