[PATCH] Make udf(4) MPSAFE and use shared lookups

So this patch is fairly minimal since udf(4) is currently read-only.  The 
changes include:

* Set VV_ROOT in udf_vget() if we ever return a vnode instead of doing it only
  in udf_root().  This matches the behavior of other operating systems and
  correctly tags the root vnode with VV_ROOT in the unlikely case that we
  create the vnode during a call to ufs_vget() that does not come from
  ufs_root().
* If the hash lookup in ufs_vget() fails, ensure an exclusive vnode lock is
  used while creating the new vnode (same as UFS).
* Allow lock recursion (XXX: not really sure this is needed actually).
* Allow shared vnode locks on non-fifos.
* Honor the requested locking flags (shared vs exclusive) instead of always
  using exclusive vnode locks during a lookup operation.
* Handle "." lookups the same way other filesystems do by just bumping the
  reference on 'dvp' rather than calling udf_vget().

http://www.FreeBSD.org/~jhb/patches/udf_mpsafe.patch

I have only verified that this compiles and would appreciate it if some folks 
could test this, preferable with INVARIANTS and DEBUG_VFS_LOCKS enabled.

-- 
John Baldwin