Add code the the krpc client so that it sets SO_SBTIMEO on the TCP socket
when the CLSET_TIMEOUT option is set. Until this patch CLSET_TIMEOUT was
never used and this patch uses it specifically for the case of NFSv4.1
mounts to DSs.
This patch gives the code better control over how long the client/MDS
will take to fail, due to an unresponsive DS and disable the DS.
This is actually a client/krpc patch and does not affect the pNFS server code.