nfs: when attempting to open a directory, fall back on normal lookup (try #5)

commit d953126 changed how nfs_atomic_lookup handles an -EISDIR return
from an OPEN call. Prior to that patch, that caused the client to fall
back to doing a normal lookup. When that patch went in, the code began
returning that error to userspace. The d_revalidate codepath however
never had the corresponding change, so it was still possible to end up
with a NULL ctx->state pointer after that.

That patch caused a regression. When we attempt to open a directory that
does not have a cached dentry, that open now errors out with EISDIR. If
you attempt the same open with a cached dentry, it will succeed.

Fix this by reverting the change in nfs_atomic_lookup and allowing
attempts to open directories to fall back to a normal lookup

Also, add a NFSv4-specific f_ops->open routine that just returns
-ENOTDIR. This should never be called if things are working properly,
but if it ever is, then the dprintk may help in debugging.

To facilitate this, a new file_operations field is also added to the
nfs_rpc_ops struct.

Cc: stable@kernel.org
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>