selftests/bpf: Fix freplace_link segfault in tailcalls prog test

There are two bpf_link__destroy(freplace_link) calls in
test_tailcall_bpf2bpf_freplace(). After the first bpf_link__destroy()
is called, if the following bpf_map_{update,delete}_elem() throws an
exception, it will jump to the "out" label and call bpf_link__destroy()
again, causing double free and eventually leading to a segfault.

Fix it by directly resetting freplace_link to NULL after the first
bpf_link__destroy() call.

Fixes: 021611d ("selftests/bpf: Add test to verify tailcall and freplace restrictions")
Signed-off-by: Tengda Wu <wutengda@huaweicloud.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Leon Hwang <leon.hwang@linux.dev>
Link: https://lore.kernel.org/bpf/20250122022838.1079157-1-wutengda@huaweicloud.com

tools/testing/selftests/bpf/prog_tests/tailcalls.c

@@ -1600,6 +1600,7 @@ static void test_tailcall_bpf2bpf_freplace(void)
 		goto out;
 
 	err = bpf_link__destroy(freplace_link);
+	freplace_link = NULL;
 	if (!ASSERT_OK(err, "destroy link"))
 		goto out;