fq - fix a (possible) deadlock

fq - fix a (possible) deadlock

 * A deadlock can occur if a lock is held around the strategy call when
   the completion is synchronous and ends up in the destruction of a
   tdio, as both will acquire the same diskctx lock.

 * Refactor the code around strategy calls so that the bios are first
   prepared, under the protection of the lock, and inserted into a local
   array. Once that is ready and the bios are ready and taken out of
   their respective queues, the locks are dropped and the strategy call
   occurs locklessly.

Reported-by: Antonio Huete (tuxillo@)