bpf: group memory related fields in struct bpf_map_memory

Group "user" and "pages" fields of bpf_map into the bpf_map_memory
structure. Later it can be extended with "memcg" and other related
information.

The main reason for a such change (beside cosmetics) is to pass
bpf_map_memory structure to charging functions before the actual
allocation of bpf_map.

Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/queue_stack_maps.c

@@ -89,7 +89,7 @@ static struct bpf_map *queue_stack_map_alloc(union bpf_attr *attr)
 
 	bpf_map_init_from_attr(&qs->map, attr);
 
-	qs->map.pages = cost;
+	qs->map.memory.pages = cost;
 	qs->size = size;
 
 	raw_spin_lock_init(&qs->lock);