[RFC]: add tracepoint to net_rx_action to note polling operations

Hey all-
	I was thinking about adding an additional tracepoint to net_rx_action,
right before the call to dev->poll().  One of the things thats missing from my
dropwatch utility is the ability to detect hardware drops, and I was thinking I
could implement that by noting when devices were being polled by napi, and at
that time querying the devices stats.

	I know however, that adding tracepoints isn't always desireable, so I
thought I would solicit opinions here to see if anyone else would find such a
tracepoint usefull, or in the event they had strong objectsions to such a point.

Thanks!
Neil