drbd: fix for spurious full sync (becoming sync target looked like invalidate)

drbd: fix for spurious full sync (becoming sync target looked like invalidate)

If a synctarget lost connection while being WFSyncUUID,
due to "state sanitizing", the attempted state change to SyncTarget
looked like an "invalidate" to after_state_ch() later,
thus caused a full sync on next handshake (Bug #318).

drbd0: PingAck did not arrive in time.
drbd0: peer( Primary -> Unknown ) conn( WFSyncUUID -> NetworkFailure ) pdsk( UpToDate -> DUnknown )

        from  : { cs:NetworkFailure ro:Secondary/Unknown ds:UpToDate/DUnknown r--- }
        to    : { cs:SyncTarget ro:Secondary/Unknown ds:Inconsistent/DUnknown r--- }
        after sanizising, resulted in
        state: { cs:NetworkFailure ro:Secondary/Unknown ds:Inconsistent/DUnknown r--- }
        drbd0: disk( UpToDate -> Inconsistent )

Fix:
don't mask state transition errors in "sanitizing",
so the requested state change to SyncTarget fails,
instead of being implicitly "remaped" to invalidate.

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