kernel - Spiff up locks a bit

kernel - Spiff up locks a bit

* Do a little optimization of _spin_lock_contested().  The critical path
  is able to avoid two atomic ops in the initialization portion of the
  contested path.

* Optimize _spin_lock_shared_contested() to use atomic_fetchadd_long()
  to add a shared-lock count instead of atomic_cmpset_long().  Shared
  spinlocks are used heavily and this will prevent a lot of unnecessary
  spinning when many cpus are using the same lock at the same time.

* Hold fdp->fd_spin across fdp->fd_cdir and fdp->fd_ncdir modifications.
  This completes other work which caches fdp->fd_ncdir and avoids having
  to obtain the spin-lock when the cache matches.

Discussed-with: Mateusz Guzik (mjg_)