net: implement netif_cond_dbg macro

For reporting things that may or may not be serious, depending on some
 condition, netif_cond_dbg will check the condition and print the report
 at either dbg (if the condition is true) or the specified level.

Suggested-by: Jon Cooper <jcooper@solarflare.com>
Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>