NFSv4: clp->cl_cons_state < 0 signifies an invalid nfs_client

If someone calls nfs_mark_client_ready(clp, status) with a negative
value for status, then that should signal that the nfs_client is no
longer valid.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>