cifs: add multichannel mount options and data structs

adds:
- [no]multichannel to enable/disable multichannel
- max_channels=N to control how many channels to create

these options are then stored in the volume struct.

- store channels and max_channels in cifs_ses

Signed-off-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>