MINOR: stream-int: add si_alloc_ibuf() to ease input buffer allocation

MINOR: stream-int: add si_alloc_ibuf() to ease input buffer allocation

This will supersed channel_alloc_buffer() while relying on it. It will
automatically adjust SI_FL_WAIT_ROOM on the stream-int depending on
success or failure to allocate this buffer.

It's worth noting that it could make sense to also set SI_FL_WANT_PUT
each time we do this to further simplify the code at user places such
as applets, but it would possibly not be easy to clean this flag
everywhere an rx operation stops.