Revert "block: Remove extra discard_alignment from hd_struct."

It was not a good idea to start dereferencing disk->queue from
the fs sysfs strategy for displaying discard alignment. We ran
into first a NULL pointer deref, and after fixing that we sometimes
see unvalid disk->queue pointer values.

Since discard is the only one of the bunch actually looking into
the queue, just revert the change.

This reverts commit 23ceb5b771.

Conflicts:
	fs/partitions/check.c

include/linux/genhd.h

@@ -100,6 +100,7 @@ struct hd_struct {
 	sector_t start_sect;
 	sector_t nr_sects;
 	sector_t alignment_offset;
+	unsigned int discard_alignment;
 	struct device __dev;
 	struct kobject *holder_dir;
 	int policy, partno;