tcp: add a common helper to debug the underlying issue

Following the commit c8770db2 ("tcp: check skb is non-NULL
in tcp_rto_delta_us()"), we decided to add a helper so that it's
easier to get verbose warning on either cases.

Link: https://lore.kernel.org/all/5632e043-bdba-4d75-bc7e-bf58014492fd@redhat.com/


Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Jason Xing <kernelxing@tencent.com>
Cc: Neal Cardwell <ncardwell@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>