bpf: Add objcg to bpf_mem_alloc

bpf: Add objcg to bpf_mem_alloc

The objcg is a bpf_mem_alloc level property since all bpf_mem_cache's
are with the same objcg. This patch made such a property explicit.
The next patch will use this property to save and restore objcg
for percpu unit allocator.

Acked-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20231222031739.1288590-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>