Only take out the periph lock when we're modifying the flags of the
softc for an async unit attention. CAM locks, sometimes, the periph
lock and other times does not. We were taking the lock always and
running into lock recursion issues on a non-recursive lock. Now we
take it selectively. It's not clear why xpt takes the lock selectively
before calling us, though, and that's still under investigation.

Reported by: avg
PR: 226510 (same panic, differnt circumstances)
Sponsored by: Netflix