selftest/bpf: Fix kprobe_multi bench test

With [1] the available_filter_functions file contains records
starting with __ftrace_invalid_address___ and marking disabled
entries.

We need to filter them out for the bench test to pass only
resolvable symbols to kernel.

[1] commit b39181f7 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")

Fixes: b39181f7 ("ftrace: Add FTRACE_MCOUNT_MAX_OFFSET to avoid adding weak function")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20220615112118.497303-5-jolsa@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

@@ -364,6 +364,9 @@ static int get_syms(char ***symsp, size_t *cntp)
 			continue;
 		if (!strncmp(name, "rcu_", 4))
 			continue;
+		if (!strncmp(name, "__ftrace_invalid_address__",
+			     sizeof("__ftrace_invalid_address__") - 1))
+			continue;
 		err = hashmap__add(map, name, NULL);
 		if (err) {
 			free(name);