perf trace: Introduce filter_loop_pids()

No change in functionality, just to make clearer that what we want when
filtering the tracer pid in a system wide tracing session is to avoid a
feedback loop.

This also paves the way for a more interesting loop avoidance algorithm,
one that tries to figure out if we are in a ssh session, xterm, etc.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-5fcttc5kdjkcyp9404ezkuy9@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>