Execsnoop's documentation says -x/--fails means "also include failed
exec()s". However it was programmed to instead skip successful execs on
-x and without -x show all - successful and unsuccessful ones.

The logic was broken in 5b47e0f8 ("execsnoop: use BPF_PERF_OUTPUT
instead of trace pipe").

Fix it.

P.S. current test_tools_smoke.py only provides basic infrastructure for
testing whether tool's BPF program won't break, without anything related
to options handling, so unfortunately the patch comes without
corresponding test.