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: NYonghong Song <yhs@fb.com>
Signed-off-by: NBo YU <tsu.yubo@gmail.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

include/linux/bpf-cgroup.h

@@ -292,7 +292,7 @@ static inline int bpf_cgroup_storage_assign(struct bpf_prog *prog,
 static inline void bpf_cgroup_storage_release(struct bpf_prog *prog,
 					      struct bpf_map *map) {}
 static inline struct bpf_cgroup_storage *bpf_cgroup_storage_alloc(
-	struct bpf_prog *prog, enum bpf_cgroup_storage_type stype) { return 0; }
+	struct bpf_prog *prog, enum bpf_cgroup_storage_type stype) { return NULL; }
 static inline void bpf_cgroup_storage_free(
 	struct bpf_cgroup_storage *storage) {}
 static inline int bpf_percpu_cgroup_storage_copy(struct bpf_map *map, void *key,

net/bpf/test_run.c

@@ -16,7 +16,7 @@
 static int bpf_test_run(struct bpf_prog *prog, void *ctx, u32 repeat,
 			u32 *retval, u32 *time)
 {
-	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE] = { 0 };
+	struct bpf_cgroup_storage *storage[MAX_BPF_CGROUP_STORAGE_TYPE] = { NULL };
 	enum bpf_cgroup_storage_type stype;
 	u64 time_start, time_spent = 0;
 	int ret = 0;