bcachefs: Use struct_size()

Use struct_size() instead of hand writing it.
This is less verbose and more robust.

While at it, prepare for the coming implementation by GCC and Clang of the
__counted_by attribute. Flexible array members annotated with __counted_by
can have their accesses bounds-checked at run-time checking via
CONFIG_UBSAN_BOUNDS (for array indexing) and CONFIG_FORTIFY_SOURCE (for
strcpy/memcpy-family functions).
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/disk_groups.c

@@ -163,8 +163,7 @@ int bch2_sb_disk_groups_to_cpu(struct bch_fs *c)
 	if (!groups)
 		return 0;
 
-	cpu_g = kzalloc(sizeof(*cpu_g) +
-			sizeof(cpu_g->entries[0]) * nr_groups, GFP_KERNEL);
+	cpu_g = kzalloc(struct_size(cpu_g, entries, nr_groups), GFP_KERNEL);
 	if (!cpu_g)
 		return -BCH_ERR_ENOMEM_disk_groups_to_cpu;
 

fs/bcachefs/super_types.h

@@ -46,7 +46,7 @@ struct bch_disk_group_cpu {
 struct bch_disk_groups_cpu {
 	struct rcu_head			rcu;
 	unsigned			nr;
-	struct bch_disk_group_cpu	entries[];
+	struct bch_disk_group_cpu	entries[] __counted_by(nr);
 };
 
 #endif /* _BCACHEFS_SUPER_TYPES_H */