[c-nsp] Output drops mysteriously appear/disappear on 3750X

[Jeff Kell]

Usually shows up (worse) on port channels.  Drops are read as a single
binary counter, and are calculated as a delta from the previously read
values.  Occasionally the port channel values are offset 2x the previous
values (individual ports versus the channel).

We've been dealing with the network monitoring "panic" alerts almost
forever.  Bug appeared in 12.2.58(?) and still waiting on fixed release
to go general release.  Have updated to current 15.x but it's still broken.

Jeff