selftests/bpf: Improve test coverage of bpf_mem_alloc.

bpf_obj_new() calls bpf_mem_alloc(), but doing alloc/free of 8 elements
is not triggering watermark conditions in bpf_mem_alloc.
Increase to 200 elements to make sure alloc_bulk/free_bulk is exercised.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/bpf/20230706033447.54696-12-alexei.starovoitov@gmail.com