bpf: Fix bpf_prog_pack build for ppc64_defconfig

bpf_prog_pack causes build error with powerpc ppc64_defconfig:

kernel/bpf/core.c:830:23: error: variably modified 'bitmap' at file scope
  830 |         unsigned long bitmap[BITS_TO_LONGS(BPF_PROG_CHUNK_COUNT)];
      |                       ^~~~~~

This is because the marco expands as:

unsigned long bitmap[((((((1UL) << (16 + __pte_index_size)) / (1 << 6))) \
     + ((sizeof(long) * 8)) - 1) / ((sizeof(long) * 8)))];

where __pte_index_size is a global variable.

Fix it by turning bitmap into a 0-length array.

Fixes: 57631054 ("bpf: Introduce bpf_prog_pack allocator")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220211024939.2962537-1-song@kernel.org

kernel/bpf/core.c

@@ -827,7 +827,7 @@ int bpf_jit_add_poke_descriptor(struct bpf_prog *prog,
 struct bpf_prog_pack {
 	struct list_head list;
 	void *ptr;
-	unsigned long bitmap[BITS_TO_LONGS(BPF_PROG_CHUNK_COUNT)];
+	unsigned long bitmap[];
 };
 
 #define BPF_PROG_MAX_PACK_PROG_SIZE	BPF_PROG_PACK_SIZE
@@ -840,7 +840,7 @@ static struct bpf_prog_pack *alloc_new_pack(void)
 {
 	struct bpf_prog_pack *pack;
 
-	pack = kzalloc(sizeof(*pack), GFP_KERNEL);
+	pack = kzalloc(sizeof(*pack) + BITS_TO_BYTES(BPF_PROG_CHUNK_COUNT), GFP_KERNEL);
 	if (!pack)
 		return NULL;
 	pack->ptr = module_alloc(BPF_PROG_PACK_SIZE);