ipv4: fib: Convert RTM_NEWROUTE and RTM_DELROUTE to per-netns RTNL.

ipv4: fib: Convert RTM_NEWROUTE and RTM_DELROUTE to per-netns RTNL.

We converted fib_info hash tables to per-netns one and now ready to
convert RTM_NEWROUTE and RTM_DELROUTE to per-netns RTNL.

Let's hold rtnl_net_lock() in inet_rtm_newroute() and inet_rtm_delroute().

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/20250228042328.96624-13-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>