MFC: 234867 and 234944

• fixed a vnode lock hang-up issue.
• fixed an incorrect lock status issue.
• fixed an incorrect lock issue of unionfs root vnode removed. (pointed out by keith)
• fixed an infinity loop issue. (pointed out by dumbbell)
• changed to do LK_RELEASE expressly when unlocked.
• fixed a unionfs_readdir math issue

Submitted by: ozawa@ongs.co.jp, Matthew Fleming <mfleming@isilon.com>