Clean-up: consumerd: reduce duplication of stream output close code

The kernel space consumer implements its own version of a stream_close
operation where it could use the common consumer code. This change
separates the tear down of the buffers (munmap, in the kernel case) from
the closing of the output stream in consumer_stream_close().

This change allows the kernel snapshot code to re-use the common
close function instead of rolling its own `finalize_snapshot_stream`.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I48e3193ceb3c15ddd8c6fcecd37ab60b793f7e66