selftests/tracing: Run sample events to clear page cache events

The tracing selftest "event-filter-function.tc" was failing because it
first runs the "sample_events" function that triggers the kmem_cache_free
event and it looks at what function was used during a call to "ls".

But the first time it calls this, it could trigger events that are used to
pull pages into the page cache.

The rest of the test uses the function it finds during that call to see if
it will be called in subsequent "sample_events" calls. But if there's no
need to pull pages into the page cache, it will not trigger that function
and the test will fail.

Call the "sample_events" twice to trigger all the page cache work before
it calls it to find a function to use in subsequent checks.

Cc: stable@vger.kernel.org
Fixes: eb50d0f2 ("selftests/ftrace: Choose target function for filter test from samples")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>