md: allow suspend_lo and suspend_hi to decrease as well as increase.

The sysfs attributes 'suspend_lo' and 'suspend_hi' describe a region
to which read/writes are suspended so that the under lying data can be
manipulated without user-space noticing.
Currently the window they describe can only move forwards along the
device.  However this is an unnecessary restriction which will cause
problems with planned developments.
So relax this restriction and allow these endpoints to move
arbitrarily.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -4016,19 +4016,24 @@ suspend_lo_store(mddev_t *mddev, const char *buf, size_t len)
 {
 	char *e;
 	unsigned long long new = simple_strtoull(buf, &e, 10);
+	unsigned long long old = mddev->suspend_lo;
 
 	if (mddev->pers == NULL || 
 	    mddev->pers->quiesce == NULL)
 		return -EINVAL;
 	if (buf == e || (*e && *e != '\n'))
 		return -EINVAL;
-	if (new >= mddev->suspend_hi ||
-	    (new > mddev->suspend_lo && new < mddev->suspend_hi)) {
-		mddev->suspend_lo = new;
+
+	mddev->suspend_lo = new;
+	if (new >= old)
+		/* Shrinking suspended region */
 		mddev->pers->quiesce(mddev, 2);
-		return len;
-	} else
-		return -EINVAL;
+	else {
+		/* Expanding suspended region - need to wait */
+		mddev->pers->quiesce(mddev, 1);
+		mddev->pers->quiesce(mddev, 0);
+	}
+	return len;
 }
 static struct md_sysfs_entry md_suspend_lo =
 __ATTR(suspend_lo, S_IRUGO|S_IWUSR, suspend_lo_show, suspend_lo_store);
@@ -4045,20 +4050,24 @@ suspend_hi_store(mddev_t *mddev, const char *buf, size_t len)
 {
 	char *e;
 	unsigned long long new = simple_strtoull(buf, &e, 10);
+	unsigned long long old = mddev->suspend_hi;
 
 	if (mddev->pers == NULL ||
 	    mddev->pers->quiesce == NULL)
 		return -EINVAL;
 	if (buf == e || (*e && *e != '\n'))
 		return -EINVAL;
-	if ((new <= mddev->suspend_lo && mddev->suspend_lo >= mddev->suspend_hi) ||
-	    (new > mddev->suspend_lo && new > mddev->suspend_hi)) {
-		mddev->suspend_hi = new;
+
+	mddev->suspend_hi = new;
+	if (new <= old)
+		/* Shrinking suspended region */
+		mddev->pers->quiesce(mddev, 2);
+	else {
+		/* Expanding suspended region - need to wait */
 		mddev->pers->quiesce(mddev, 1);
 		mddev->pers->quiesce(mddev, 0);
-		return len;
-	} else
-		return -EINVAL;
+	}
+	return len;
 }
 static struct md_sysfs_entry md_suspend_hi =
 __ATTR(suspend_hi, S_IRUGO|S_IWUSR, suspend_hi_show, suspend_hi_store);