MFC r273112: Set the DROP_EN bit before the RX queue is brought up and active.

He noticed issues setting this bit in SRRCTL after the queue was up,
so doing it from the sysctl handler isn't enough and may not actually
work correctly.

This commit doesn't remove the sysctl path or try to change its
behaviour. I'll talk with others about how to finish fixing that
before I tackle that.

PR: kern/194311
Submitted by: luigi
MFC after: 3 days
Sponsored by: Norse Corp, Inc