Fix a LOR issue with new NFS server.

In nfsrvd_rename(), the caller (RPC handler, via nfsrvd_dorpc)
locks and references the source directory vnode ('dp' in the
context) for us.

In the context, we are going to call VOP_FHTOVP, which is
expected to return the resuting vnode with a lock, to find
the target directory's vnode via a file handle.  The
underlying code is expected to lock that vnode before
returning.

Because we already have held the source directory's vnode,
grabbing the target's directory vnode can be harmful because
if there is some hierarchy relationship between the two,
the second lock may have been grabbed in the wrong order.

Solve this by releasing the source directory vnode's lock early.
We do so right before calling any VFS methods that would
grab more locks (we still have reference at this point which
would prevent the memory to get actually free'ed after we
released the lock).

Submitted by:	Rick Macklem <rmacklem@FreeBSD.org>
Ticket:		#3867