selftests/resctrl: Fix uninitialized .sa_flags

signal_handler_unregister() calls sigaction() with uninitializing
sa_flags in the struct sigaction.

Make sure sa_flags is always initialized in signal_handler_unregister()
by initializing the struct sigaction when declaring it. Also add the
initialization to signal_handler_register() even if there are no know
bugs in there because correctness is then obvious from the code itself.

Fixes: 73c55fa5

 ("selftests/resctrl: Commonize the signal handler register/unregister for all tests")
Suggested-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: <stable@vger.kernel.org>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>