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>
diff --git a/Documentation/md.txt b/Documentation/md.txt
index e06cc59..1da9d1b 100644
--- a/Documentation/md.txt
+++ b/Documentation/md.txt
@@ -347,7 +347,7 @@
         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