iflib: prefer simple_tx over mp_ring

[gallatin]

Summary

In preparation for removing mp_ring, enable simple_tx by default.

Users encountering bugs are still able to set the tunable:
net.iflib.prefer_mpring="1" at boot, but are encouraged
to report those bugs so that I can fix them.

My plan is to remove mp_ring assuming no major bugs are found in
simple_tx after a month or so.

The motivation is that that simple_tx has better performance in all scenarios
I've been able to test and it is easier to understand and maintain.

[netchild]

FWIW: I run simple_tx since our discussion about the watchdog on the system which triggered my watchdog changes. I can not tell anything about the performance (heavily under-equiped memory there), but I can not see any obvious ill-effects.

[kbowling]

We also have over a year to investigate and fix any issues before the next release. Since there are no known regressions currently, makes sense to me to start baking.