md: Tidy up rdev_size_store a bit:

 - used strict_strtoull in place of simple_strtoull
 - use my_mddev in place of rdev->mddev (they have the same value)
and more significantly,
 - don't adjust mddev->size to fit, rather reject changes which make
   rdev->size smaller than mddev->size

Adjusting mddev->size is a hangover from bind_rdev_to_array which
does a similar thing.  But it really is a better design to insist that
mddev->size is set as required, then the rdev->sizes are set to allow
for that.  The previous way invites confusion.
Signed-off-by: NeilBrown <neilb@suse.de>

Documentation/md.txt

@@ -347,7 +347,7 @@ Each directory contains:
         for storage of data.  This will normally be the same as the
 	component_size.  This can be written while assembling an
         array.  If a value less than the current component_size is
-        written, component_size will be reduced to this value.
+        written, it will be rejected.
 
 
 An active md device will also contain and entry for each active device

drivers/md/md.c

@@ -2101,16 +2101,17 @@ static int overlaps(sector_t s1, sector_t l1, sector_t s2, sector_t l2)
 static ssize_t
 rdev_size_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 {
-	char *e;
-	unsigned long long size = simple_strtoull(buf, &e, 10);
+	unsigned long long size;
 	unsigned long long oldsize = rdev->size;
 	mddev_t *my_mddev = rdev->mddev;
 
-	if (e==buf || (*e && *e != '\n'))
+	if (strict_strtoull(buf, 10, &size) < 0)
+		return -EINVAL;
+	if (size < my_mddev->size)
 		return -EINVAL;
 	if (my_mddev->pers && rdev->raid_disk >= 0) {
-		if (rdev->mddev->persistent) {
-			size = super_types[rdev->mddev->major_version].
+		if (my_mddev->persistent) {
+			size = super_types[my_mddev->major_version].
 				rdev_size_change(rdev, size);
 			if (!size)
 				return -EBUSY;
@@ -2118,12 +2119,12 @@ rdev_size_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 			size = (rdev->bdev->bd_inode->i_size >> 10);
 			size -= rdev->data_offset/2;
 		}
-		if (size < rdev->mddev->size)
+		if (size < my_mddev->size)
 			return -EINVAL; /* component must fit device */
 	}
 
 	rdev->size = size;
-	if (size > oldsize && rdev->mddev->external) {
+	if (size > oldsize && my_mddev->external) {
 		/* need to check that all other rdevs with the same ->bdev
 		 * do not overlap.  We need to unlock the mddev to avoid
 		 * a deadlock.  We have already changed rdev->size, and if
@@ -2165,8 +2166,6 @@ rdev_size_store(mdk_rdev_t *rdev, const char *buf, size_t len)
 			return -EBUSY;
 		}
 	}
-	if (size < my_mddev->size || my_mddev->size == 0)
-		my_mddev->size = size;
 	return len;
 }