cifs: use the least loaded channel for sending requests

cifs: use the least loaded channel for sending requests

Till now, we've used a simple round robin approach to
distribute the requests between the channels. This does
not work well if the channels consume the requests at
different speeds, even if the advertised speeds are the
same.

This change will allow the client to pick the channel
with least number of requests currently in-flight. This
will disregard the link speed, and select a channel
based on the current load of the channels.

For cases when all the channels are equally loaded,
fall back to the old round robin method.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Reviewed-by: Paulo Alcantara (SUSE) <pc@cjr.nz>
Signed-off-by: Steve French <stfrench@microsoft.com>