drbd: Only print sanitize state's warnings, if the state change happens

The reason for this change is that, with when doing
'drbdadm invalidate' on a disconnected resource caused
an "implicitly set pdsk from UpToDate to DUnknown" message,
which was missleading.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>