Fix a race with shared lookups on UFS. If the the dirhash code reached the

Fix a race with shared lookups on UFS.  If the the dirhash code reached the
cap on memory usage, then shared LOOKUP operations could start free'ing
dirhash structures.  Without these fixes, concurrent free's on the same
directory could result in one of the threads blocked on a lock in a dirhash
structure free'd by the other thread.
- Replace the lockmgr lock in the dirhash structure with an sx lock.
- Use a reference count managed with ufsdirhash_hold()/drop() to determine
  when to free the dirhash structures.  The directory i-node holds a
  reference while the dirhash is attached to an i-node.  Code that wishes
  to lock the dirhash while holding a shared vnode lock must first
  acquire a private reference to the dirhash while holding the vnode
  interlock before acquiring the dirhash sx lock.  After acquiring the sx
  lock, it drops the private reference after checking to see if the
  dirhash is still used by the directory i-node.