Fix NFSv4.1 client side handling of "soft,retrans=2" mounts.

Normally "soft,retrans=2" cannot be safely used on NFSv4 mounts, since
the RPC can fail and leave the open/lock state in an undefined state.
Doing I/O on a pNFS DS is an exception to this, since no open/lock state
is maintained on the DS server.
It is useful to do "soft,retrans=2" connections to a DS when it is mirrored,
so that the client can detect failure of the DS. As such, mounts from the MDS
to the DSs should use these mount options when mirroring is enabled.
However, the NFSv4.1 client still leaves the session in an undefined state
when this happens.
This patch fixes the problem by setting the session defunct, so it will
no longer be used.
The patch also sets "retries=2" on the connections done by the client to
a DS, which is the internal equivalent of "soft,retrans=2".
The client does not know if the server implements mirroring at connection
time, but always doing this should be safe, since it will fall back on doing
I/O via the MDS as a proxy when there is a failure doing an I/O RPC to the DS.

This patch should not affect non-pNFS client mounts.

MFC after: 2 weeks