kbuffer: Add kbuffer_subbuffer() API

Add a way to retrieve the last loaded subbuffer to the kbuffer.

The kbuffer_subbuffer() will return a pointer to the last loaded subbuffer or
NULL if none was ever loaded.

Link: https://lore.kernel.org/linux-trace-devel/20231224191813.1076074-6-rostedt@goodmis.org

Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
4 files changed