bebob: fix memory leak/buffer over run of snd_bebob_stream_map()

This function don't kfree() even if it does kmalloc().

And with some devices, this function does buffer over run to write on outside
of buffer for MIDI channel mapping.

This function is not referred from the external so changed local static function.

Additionally, 'cluster' is also used in IEC 61883-6. To avoid misunderstanding,
I pick up 'section' from BridgeCo document. Then rename this function without
'cluster', with 'section'.