NFSv4/pNFS: Don't call _nfs4_pnfs_v3_ds_connect multiple times

stable inclusion
from linux-4.19.198
commit e3eeeaed0a54f664b025f290a819897af8a1eccc

--------------------------------

[ Upstream commit f46f8493 ]

After we grab the lock in nfs4_pnfs_ds_connect(), there is no check for
whether or not ds->ds_clp has already been initialised, so we can end up
adding the same transports multiple times.

Fixes: fc821d59 ("pnfs/NFSv4.1: Add multipath capabilities to pNFS flexfiles servers over NFSv3")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>