NFSv4.1 another fix for EXCHGID4_FLAG_USE_PNFS_DS for DS server

Previously in order to mark the communication with the DS server,
we tried to use NFS_CS_DS in cl_flags. However, this flag would
only be saved for the DS server and in case where DS equals MDS,
the client would not find a matching nfs_client in nfs_match_client
that represents the MDS (but is also a DS).

Instead, don't rely on the NFS_CS_DS but instead use NFS_CS_PNFS.

Fixes: 379e4adf ("NFSv4.1: fixup use EXCHGID4_FLAG_USE_PNFS_DS for DS server")
Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>