sanlock: allow setting max_sectors_kb
    
    Allow sanlock to set this sysfs file:
      /sys/dev/block/<major>:<minor>/queue/max_sectors_kb
    
    to optimize read io's sent to a storage device.  By default
    this value is 512K which means each 1M read is split into
    two 512K reads that are sent to storage.  If set to 1024,
    then each 1M read is sent to storage without being split.
    
    The sanlock behavior is controlled by the max_sectors_kb
    setting in sanlock.conf:
    
    max_sectors_kb = ignore
      sanlock doesn't set it (the current default).
    
    max_sectors_kb = align
      sanlock will set it to the lockspace align size (1M/2M/4M/8M)
      when adding the lockspace.
    
    max_sectors_kb = <num_kb>
      sanlock will set it to <num_kb> for each lockspace.