nfsclient: Fix ReadDS/WriteDS/CommitDS nfsstats RPC counts for a NFSv3 DS

During a recent virtual NFSv4 testing event, a bug in the FreeBSD client
was detected when doing I/O DS operations on a Flexible File Layout pNFS
server. For an NFSv3 DS, the Read/Write/Commit nfsstats were incremented
instead of the ReadDS/WriteDS/CommitDS counts.
This patch fixes this.

Only the RPC counts reported by nfsstat(1) were affected by this bug,
the I/O operations were performed correctly.

MFC after: 2 weeks