kbuffer: Add kbuffer_dup()

Add the function kbuffer_dup() that will duplicate a current kbuffer structure
that can be used separately on the same subbuffer or load a new buffer on it.

Link: https://lore.kernel.org/linux-trace-devel/20231224191813.1076074-5-rostedt@goodmis.org

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