Usage: int ddu_put_fev(struct fifo_entry fev)
Returns: 0 if good status, -1 if bad status, >0 if interrupted system call
Puts a fifo entry fev back into the DD system. This routine is called
by producers to add a new fifo entry or by consumers that are finished
handling the event obtained by a previous call to ddu_get_fev(struct fifo_entry *fev) .