md: Fix rdev_size_store with size == 0

commit 7d3c6f8717ee6c2bf6cba5fa0bda3b28fbda6015 upstream

Fix rdev_size_store with size == 0.
size == 0 means to use the largest size allowed by the
underlying device and is used when modifying an active array.

This fixes a regression introduced by
 commit d7027458d68b2f1752a28016dcf2ffd0a7e8f567

Signed-off-by: Chris Webb <chris@arachsys.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>