bcachefs: Fix nested transaction restart handling in bch2_bucket_gens_init()

Nested transaction restart handling is typically best avoided; when the
inner context handles a transaction restart it invalidates the outer
transaction context, so we need to make sure to return a
transaction_restart_nested error.

This code wasn't doing that, and hit the assertion in
for_each_btree_key() that checks for that via trans->restart_count.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -532,13 +532,13 @@ int bch2_bucket_gens_init(struct bch_fs *c)
 		u8 gen = bch2_alloc_to_v4(k, &a)->gen;
 		unsigned offset;
 		struct bpos pos = alloc_gens_pos(iter.pos, &offset);
+		int ret2 = 0;
 
 		if (have_bucket_gens_key && bkey_cmp(iter.pos, pos)) {
-			ret = commit_do(trans, NULL, NULL,
-					BCH_TRANS_COMMIT_no_enospc,
-				bch2_btree_insert_trans(trans, BTREE_ID_bucket_gens, &g.k_i, 0));
-			if (ret)
-				break;
+			ret2 =  bch2_btree_insert_trans(trans, BTREE_ID_bucket_gens, &g.k_i, 0) ?:
+				bch2_trans_commit(trans, NULL, NULL, BCH_TRANS_COMMIT_no_enospc);
+			if (ret2)
+				goto iter_err;
 			have_bucket_gens_key = false;
 		}
 
@@ -549,7 +549,8 @@ int bch2_bucket_gens_init(struct bch_fs *c)
 		}
 
 		g.v.gens[offset] = gen;
-		0;
+iter_err:
+		ret2;
 	}));
 
 	if (have_bucket_gens_key && !ret)