ioat: refcnt users so we can drain them at detach

We only need to borrow a mutex for the drain sleep and the 0->1
transition, so just reuse an existing one for now.

The wchan is arbitrary. Using refcount itself would have required
__DEVOLATILE(), so use the lock's address instead.

Different uses are tagged by kind, although we only do anything with
that information in INVARIANTS builds.

Sponsored by: EMC / Isilon Storage Division