ftrace: Fix ftrace_trace_task return value

I was attempting to use pid filtering with function_graph, but it wasn't
allowing anything to make it through.  Turns out ftrace_trace_task
returns false if ftrace_ignore_pid is not-empty, which isn't correct
anymore.  We're now setting it to FTRACE_PID_IGNORE if we need to ignore
that pid, otherwise it's set to the pid (which is weird considering the
name) or to FTRACE_PID_TRACE.  Fix the check to check for !=
FTRACE_PID_IGNORE.  With this we can now use function_graph with pid
filtering.

Link: https://lkml.kernel.org/r/20200725005048.1790-1-josef@toxicpanda.com

Fixes: 717e3f5e

 ("ftrace: Make function trace pid filtering a bit more exact")
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>