Set CLSET_TIMEOUT on TCP connections to pNFS DSs.

Use CLSET_TIMEOUT to set the timeout for connections to DSs instead of
specifying a timeout on each RPC. This is done so that SO_SNDTIMEO
is set on the TCP socket as well as specifying a time limit when
waiting for an RPC reply. Useful if the send queue for the TCP
connection has become constipated, due to a failed DS.
The choice of lease_duration / 4 is fairly arbitrary, but seems to work
ok, with a lower bound of 10sec.
For client connections to a DS, set the retry limit to vfs.nfsd.dsretries,
which is 2 by default.
This patch should only affect pNFS connections to DSs.
This patch requires r336542.

MFC after: 2 weeks