pNFS/flexfiles: Check the result of nfs4_pnfs_ds_connect

The check in nfs4_ff_layout_prepare_ds() seems to be missing.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Fixes: a33e4b03 ("pNFS: return status from nfs4_pnfs_ds_connect")
Cc: Weston Andros Adamson <dros@primarydata.com>
Cc: stable@vger.kernel.org # v4.11