dsched_fq - Overhaul locking

dsched_fq - Overhaul locking

* use lockmgr lock for FQP lock, as some strategy ops can sleep while
  acquiring another lock (CAM SIM lock, for example).

* reduce overall locking when it isn't really required, mainly during
  deallocation (losing last ref) of objects. The locking is only
  explicitly required to protect the internal TAILQs.

* NOTE: this is an _attempt_ to fix some unidentified deadlocks that have
  been reported occasionally. While it shouldn't happen, be aware that
  this might explode.

Reported-by: Antonio Huete, Jan Lentfer