bcachefs: Annotate struct bucket_gens with __counted_by()

bcachefs: Annotate struct bucket_gens with __counted_by()

Add the __counted_by compiler attribute to the flexible array member b
to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size() to calculate the number of bytes to be allocated.

Update bucket_gens->nbuckets and bucket_gens->nbuckets_minus_first when
resizing.

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>