ipv4: fib: Hold rtnl_net_lock() in ip_rt_ioctl().

ipv4: fib: Hold rtnl_net_lock() in ip_rt_ioctl().

ioctl(SIOCADDRT/SIOCDELRT) calls ip_rt_ioctl() to add/remove a route in
the netns of the specified socket.

Let's hold rtnl_net_lock() there.

Note that rtentry_to_fib_config() can be called without rtnl_net_lock()
if we convert rtentry.dev handling to RCU later.

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/20250228042328.96624-11-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>