DCB state is changed prior to polling operation

In poll.c functions poll_add_dcb and poll_remove_dcb, an attempt is made to change the state of the DCB prior to carrying out the operation on the poll list. The state change may fail, in which case the operation is not done. However, if the state change succeeds it is still possible for the operation on the poll list to fail; in this case the state of the DCB is not reverted to what it was originally. Reversion may not be possible in all cases, based on the logic for state transitions. Leaving the DCB in an incorrect state after a failure of a polling list operation has unknown but probably detrimental effects. Correcting this problem may also have unknown detrimental effects.