[PATCH] md: simplify checking of available size when resizing an array

When "mdadm --grow --size=xxx" is used to resize an array (use more or less of
each device), we check the new siza against the available space in each
device.

We already have that number recorded in rdev->size, so calculating it is
pointless (and wrong in one obscure case).

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>