ftrace: Only disable ftrace_enabled to test buffer in selftest

The ftrace_enabled variable is set to zero in the self tests to keep
delayed functions from being traced and messing with the checks. This
only needs to be done when the checks are being performed, otherwise,
if ftrace_enabled is off when calls back to the utility that is being
tested, it can cause errors to happen and the tests can fail with
false positives.

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