md: fix regression with re-adding devices to arrays with no metadata

Commit 1a855a06 (2.6.37-rc4) fixed a problem where devices were
re-added when they shouldn't be but caused a regression in a less
common case that means sometimes devices cannot be re-added when they
should be.

In particular, when re-adding a device to an array without metadata
we should always access the device, but after the above commit we
didn't.

This patch sets the In_sync flag in that case so that the re-add
succeeds.

This patch is suitable for any -stable kernel to which 1a855a06

 was
applied.

Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>