pNFS/flexfiles: Set reasonable default retrans values for the data channel

Prior to this patch, the retrans value was set at 5, meaning that we
could see a maximum retransmission timeout value of more than 6 minutes.
That's a tad high for NFSv3 where the protocol does allow the server to
drop requests at any time.

Since this is a data channel, let's just set retrans to 0, and the default
timeout to 60s. The user can continue to adjust these defaults using the
dataserver_retrans and dataserver_timeo module parameters.

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