bpf: fix warning about using plain integer as NULL

Sparse warning below:

sudo make C=2 CF=-D__CHECK_ENDIAN__ M=net/bpf/
CHECK   net/bpf//test_run.c
net/bpf//test_run.c:19:77: warning: Using plain integer as NULL pointer
./include/linux/bpf-cgroup.h:295:77: warning: Using plain integer as NULL pointer

Fixes: 8bad74f9

 ("bpf: extend cgroup bpf core to allow multiple cgroup storage types")
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Bo YU <tsu.yubo@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>