selftests/ftrace: Add ringbuffer size changing testcase

Add a testcase for changing ringbuffer size. This tests
not only ringbuffer size but also tests the imbalance
per-cpu buffer size change too.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: Shuah Khan (Samsung OSG) <shuah@kernel.org>