NFS: Reduce use of uncached readdir

When reading a very large directory, we want to try to keep the page
cache up to date if doing so is inexpensive. With the change to allow
readdir to continue reading even when the cache is incomplete, we no
longer need to fall back to uncached readdir in order to scale to large
directories.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>