[rfc] migrate lagg to an rmlock

[Adrian Chadd]

Hihi,

There's two parts to my patch:

* one is migrating the rwlock to rmlock - not because of counters, but
because the lock is protecting consistency when changing the lagg config
* one is adding a new lock specifically to ensure that the callout is
atomically created/called/destroyed

The latter has nothing to do with the actual counters - they're already
using the atomic counter API, so the lock doesn't need to be held just to
read them for _counter_ consistency. It's just held to make sure the
callout is never called parallel to the destruction of the lagg interface.



-adrian