ftrace: do not process freed records

Impact: keep from converting freed records

When the tracer is started or stopped, it converts all code pointed
to by the saved records into callers to ftrace or nops. When modules
are unloaded, their records are freed, but they still exist within
the record pages.

This patch changes the code to skip over freed records.

Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>