blkif: Fix alignment description for discard request

The discard feature have an other xenstore node to described the size
of the blocks than can be discarded, "discard-granularity", which
default to "sector-size" when absent as noted in the properties and in
note 4. So discard request should be aligned on this value.

Fixes: 221f2748e8 ("blkif: reconcile protocol specification with in-use implementations")
Signed-off-by: Anthony PERARD <anthony.perard@vates.tech>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

xen/include/public/io/blkif.h

@@ -668,7 +668,7 @@ typedef struct blkif_request blkif_request_t;
  *
  * The 'sector_number' field is in units of 512b, despite the value of the
  * 'sector-size' xenstore node.  Note however that the offset in
- * 'sector_number' must be aligned to 'sector-size'.
+ * 'sector_number' must be aligned to 'discard-granularity'.
  */
 struct blkif_request_discard {
     uint8_t        operation;    /* BLKIF_OP_DISCARD                     */