MFC: r224554

MFC: r224554
Fix rename in the new NFS server so that it does not require a
recursive vnode lock on the directory for the case where the
new file name is in the same directory as the old one. The patch
handles this as a special case, recognized by the new directory
having the same file handle as the old one and just VREF()s the old
dir vnode for this case, instead of doing a second VFS_FHTOVP() to get it.
This is required so that the server will work for file systems like
msdosfs, that do not support recursive vnode locking.
This problem was discovered during recent testing by pho@
when exporting an msdosfs file system via the new NFS server.

git-svn-id: svn://svn.freebsd.org/base/stable/8@224860 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f