NFSv4.1: use EXCHGID4_FLAG_USE_PNFS_DS for DS server

After receiving the location(s) of the DS server(s) in the
GETDEVINCEINFO, create the request for the clientid to such
server and indicate that the client is connecting to a DS.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>