tsync: only one of the two siblings should fail

In two_siblings_not_under_filter, only 1 sibling should fail since the other
will be pulled into seccomp mode.

Signed-off-by: Kees Cook <keescook@chromium.org>
Merge pull request #4 from kees/tsync

tsync: only one of the two siblings should fail
tsync: unlock mutex on failure

Testing the there-is-no-seccomp case showed the children need to
unlock on fatal errors.

Signed-off-by: Kees Cook <keescook@chromium.org>
Merge pull request #5 from kees/tsync

tsync: unlock mutex on failure