ipv4: fib: Hold rtnl_net_lock() for ip_fib_net_exit().

ip_fib_net_exit() requires RTNL and is called from fib_net_init()
and fib_net_exit_batch().

Let's hold rtnl_net_lock() before ip_fib_net_exit().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250228042328.96624-10-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>