kernel - Reduce spinning on shared spinlocks

kernel - Reduce spinning on shared spinlocks

* Improve spinlock performance by removing unnecessary extra reads,
  using atomic_fetchadd_int() to avoid a cmpxchg loop, and allowing
  the SHARED flag to remain soft-set on the 1->0 transition.

* The primary improvement here is that multiple cpu's obtaining the
  same shared spinlock can now do so via a single atomic_fetchadd_int(),
  whereas before we had multiple atomics and cmpxchg loops.  This does not
  remove the cacheline ping-pong but it significantly reduces unnecessary
  looping when multiple cpu cores are heavily loading the same shared spin
  lock.

* Trade-off is against the case where a spinlock's use-case switches from
  shared to exclusive or back again, which requires an extra atomic op to
  deal with.  This is not a common case.

* Remove spin->countb debug code, it interferes with hw cacheline operations
  and is no longer desireable.

Discussed-with: Mateusz Guzik (mjg_)