NFSv4.1: Kill nfs4_ds_disconnect()

There is nothing to prevent another thread from dereferencing ds->ds_clp
during or after the call to nfs4_ds_disconnect(), and Oopsing due to the
resulting NULL pointer.

Instead, we should just rely on filelayout_mark_devid_invalid() to keep
us out of trouble by avoiding that deviceid.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>