NFS: Shut down the nfs_client only after all the superblocks

The nfs_client manages state for all the superblocks in the
"cl_superblocks" list, so it must not be shut down until all of them are
gone.

Fixes: 7d3e26a0 ("NFS: Cancel all existing RPC tasks when shutdown")
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>