NFSv4.1: fixup use EXCHGID4_FLAG_USE_PNFS_DS for DS server

This patches fixes commit 51d674a5 "NFSv4.1: use
EXCHGID4_FLAG_USE_PNFS_DS for DS server", purpose of that
commit was to mark EXCHANGE_ID to the DS with the appropriate
flag.

However, connection to MDS can return both EXCHGID4_FLAG_USE_PNFS_DS
and EXCHGID4_FLAG_USE_PNFS_MDS set but previous patch would only
remember the USE_PNFS_DS and for the 2nd EXCHANGE_ID send that
to the MDS.

Instead, just mark the pnfs path exclusively.

Fixes: 51d674a5

 ("NFSv4.1: 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>