complete_rqe: Remove a race condition in RAID-4 and RAID-5 where a
request could be deallocated before the top half had finished
issuing it. The problem seems only to happen with IDE drives
and vn devices, but theoretically it could happen with any
drive. This is the most important part of a possible series
of fixes designed to remove race conditions without locking
out interrupts for longer than absolutely necessary.