selftest/bpf: Fix memory leak in kprobe_multi_test

The get_syms() function in kprobe_multi_test.c does not free the string
memory allocated by sscanf correctly. Fix it.

Fixes: 5b6c7e5c ("selftests/bpf: Add attach bench test")
Signed-off-by: Xu Kuohai <xukuohai@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://lore.kernel.org/bpf/20221011120108.782373-5-xukuohai@huaweicloud.com