NFSv4/pNFS: Return an error if _nfs4_pnfs_v3_ds_connect can't load NFSv3

Currently we fail to return an error if the NFSv3 module failed to load
when we're trying to connect to a pNFS data server.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>