MINOR: buffer: split bi_contig_data() into ci_contig_data and b_config_data()

MINOR: buffer: split bi_contig_data() into ci_contig_data and b_config_data()

This function was sometimes used from a channel and sometimes from a buffer.
In both cases it requires knowledge of the size of the output data (to skip
them). Here the split ensures the channel can deal with this point, and that
other places not having output data can continue to work.