MINOR: filters: rework filter iteration for channel related callback functions

MINOR: filters: rework filter iteration for channel related callback functions

Multiple channel related functions have the same construction: they use
list_for_each_entry() to work on a given filter from the stream+channel
combination. In future commits we will try to use filter list from
dedicated channel list instead of the stream one, thus in this patch we
need as a prerequisite to implement and use the flt_list_{start,next} API
to iterate over filter list, giving the API the responsibility to iterate
over the correct list depending on the context, while the calling function
remains free to use the iteration construction it needs. This way we will
be able to easily change the way we iterate over filter list without
duplicating the code for requests and responses.