fib:fix BUG_ON in fib_nl_newrule when add new fib rule

add new fib rule can cause BUG_ON happen
the reproduce shell is
ip rule add pref 38
ip rule add pref 38
ip rule add to 192.168.3.0/24 goto 38
ip rule del pref 38
ip rule add to 192.168.3.0/24 goto 38
ip rule add pref 38

then the BUG_ON will happen
del BUG_ON and use (ctarget == NULL) identify whether this rule is unresolved

Signed-off-by: Gao feng <gaofeng@cn.fujitsu.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>