drbd: Becoming sync target may not happen out of < C_WF_REPORT_PARAMS

This patch is acutally a necessary addendum to the patch
"fix for spurious full sync (becoming sync target looked like invalidate)"

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