selftests/bpf: Fix memory leak in test_ima

The allocated ring buffer is never freed, do so in the cleanup path.

Fixes: f446b570 ("bpf/selftests: Update the IMA test to use BPF ring buffer")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211028063501.2239335-9-memxor@gmail.com