ftrace: Do not run selftest if command line parameter is set

If the kernel command line ftrace filter parameters are set
(ftrace_filter or ftrace_notrace), force the function self test to
pass, with a warning why it was forced.

If the user adds a filter to the kernel command line, it is assumed
that they know what they are doing, and the self test should just not
run instead of failing (which disables function tracing) or clearing
the filter, as that will probably annoy the user.

If the user wants the selftest to run, the message will tell them why
it did not.

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