fib_trie: call fib_table_flush_external under RTNL

Move rtnl_lock() before the call to fib4_rules_exit so that
fib_table_flush_external is called under RTNL.

Fixes: 104616e7

 ("switchdev: don't support custom ip rules, for now")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>