Problem with DISCARD and RAID5

Hi Shaohua,
 I've been doing some testing and discovered a problem with your discard
 support for RAID5.

 The code in blkdev_issue_discard assumes that the 'granularity' is a power
 of 2, and for example subtracts 1 to get a mask.

 However RAID5 sets the granularity to be the stripe size which often is not
 a power of two.  When this happens you can easily get into an infinite loop.

 I suspect that to make this work properly, blkdev_issue_discard will need to
 be changed to allow 'granularity' to be an arbitrary value.
 When it is a power of two, the current masking can be used.
 When it is anything else, it will need to use sector_div().

 Could you look into this please?

Thanks,
NeilBrown