child-sa: Use SA matching mark as SA set mark if the latter is %same

For inbound processing, it can be rather useful to apply the mark to the
packet in the SA, so the associated policy with that mark implicitly matches.
When using %unique as match mark, we don't know the mark beforehand, so
we most likely want to set the mark we match against.

src/libstrongswan/tests/suites/test_utils.c

@@ -916,6 +916,12 @@ static struct {
 		{ 0, 0 }},
 	{"%unique-foo/0xffffffff",	FALSE,	MARK_OP_UNIQUE,
 		{ 0, 0 }},
+	{"%same",					TRUE,	MARK_OP_SAME,
+		{ MARK_SAME, 0xffffffff }},
+	{"%same/0x0000ffff",		TRUE,	MARK_OP_SAME,
+		{ MARK_SAME, 0x0000ffff }},
+	{"%%same",					FALSE,	MARK_OP_NONE,
+		{ 0, 0 }},
 };
 
 START_TEST(test_mark_from_string)