NFS: Fix dentry revalidation on NFSv4 lookup

We need to ensure that inode and dentry revalidation occurs correctly
on reopen of a file that is already open. Currently, we can end up
not revalidating either in the case of NFSv4.0, due to the 'cached open'
path.
Let's fix that by ensuring that we only do cached open for the special
cases of open recovery and delegation return.

Reported-by: Stan Hu <stanhu@gmail.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>