lttng: lib ring buffer clients

Each lttng buffer configuration (discard mode, overwrite mode, mmap
support, splice support, per-cpu buffers, global buffer for metadata) is
a lib ring buffer client.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>