Fix segfault caused by using rdev_for_each instead of rdev_for_each_safe

Commit dafb20fa mistakenly replaced a safe
iterator with an unsafe one when making some macro changes.
Signed-off-by: NJonathan Brassow <jbrassow@redhat.com>
Signed-off-by: NNeilBrown <neilb@suse.de>