cifs: Make devname param optional in cifs_compose_mount_options()

If we only want to get the mount options strings, do not return the
devname.

For DFS failover, we'll be passing the DFS full path down to
cifs_mount() rather than the devname.

Signed-off-by: Paulo Alcantara <palcantara@suse.de>
Reviewed-by: Aurelien Aptel <aaptel@suse.com>
Signed-off-by: Steve French <stfrench@microsoft.com>