ipv4: use WARN_ON_ONCE() in ip_rt_bug()

ipv4: use WARN_ON_ONCE() in ip_rt_bug()

It turns out ip_rt_bug() can be called more than expected.

syzbot will still panic (because of panic_on_warn=1), but non debug
kernels will no longer die while repeating stack traces on the console.

Fixes: c378a9c019cf ("ipv4: Give backtrace in ip_rt_bug().")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/20260519193248.4018872-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>