nfscl: Fix NFSv4.1/4.2 pnfs mounts using nconnect

When a mount with the "pnfs" and "nconnect" options specified
does an I/O operation, it erroneously uses a TCP connection
to the MDS when it is meant to be a DS operation and, as such,
needs to use a TCP connection to the DS. This patch fixes this.

When the "pnfs" and "nconnect" options are specified for a
NFSv4.1/4.2 mount, there probably should be N connections
established to each DS for I/O RPCs. This is a fair amount
of work and may be done in a future commit.

This problem was found during a recent IETF NFSv4 working
group testing event.

MFC after: 2 weeks