bcachefs: Fix ca->oldest_gen allocation

The ca->oldest_gen array needs to be the same size as the bucket_gens
array; ca->mi.nbuckets is updated with only state_lock held, not
gc_lock, so bch2_gc_gens() could race with device resize and allocate
too small of an oldest_gens array.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_gc.c

@@ -1954,19 +1954,17 @@ int bch2_gc_gens(struct bch_fs *c)
 	trans = bch2_trans_get(c);
 
 	for_each_member_device(ca, c, i) {
-		struct bucket_gens *gens;
+		struct bucket_gens *gens = bucket_gens(ca);
 
 		BUG_ON(ca->oldest_gen);
 
-		ca->oldest_gen = kvmalloc(ca->mi.nbuckets, GFP_KERNEL);
+		ca->oldest_gen = kvmalloc(gens->nbuckets, GFP_KERNEL);
 		if (!ca->oldest_gen) {
 			percpu_ref_put(&ca->ref);
 			ret = -BCH_ERR_ENOMEM_gc_gens;
 			goto err;
 		}
 
-		gens = bucket_gens(ca);
-
 		for (b = gens->first_bucket;
 		     b < gens->nbuckets; b++)
 			ca->oldest_gen[b] = gens->b[b];