[rfc] migrate lagg to an rmlock

Hi,

I'd like to commit this to -10. It migrates the if_lagg locking from a rw
lock to a rm lock. We see a bit of contention between the transmit and
receive sides of lagg during traffic loads (10+ gigabit per second.) Using
rmlocks eliminate this.

http://people.freebsd.org/~adrian/netflix/20130819-lagg-rmlock-1.diff

Thanks,


-adrian