bcachefs: Mark new snapshots earlier in create path

This fixes a null ptr deref when creating new snapshots:
bch2_create_trans() will lookup the subvolume and find the _new_
snapshot in the BCH_CREATE_SUBVOL path that's being created in that
transaction.

We have to call bch2_mark_snapshot() earlier so that it's properly
initialized, instead of leaving it for transaction commit.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/subvolume.c

@@ -513,7 +513,9 @@ int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
 		n->v.pad	= 0;
 		SET_BCH_SNAPSHOT_SUBVOL(&n->v, true);
 
-		ret   = bch2_trans_update(trans, &iter, &n->k_i, 0);
+		ret   = bch2_trans_update(trans, &iter, &n->k_i, 0) ?:
+			bch2_mark_snapshot(trans, BTREE_ID_snapshots, 0,
+					   bkey_s_c_null, bkey_i_to_s_c(&n->k_i), 0);
 		if (ret)
 			goto err;
 
@@ -540,7 +542,7 @@ int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
 		n->v.children[1] = cpu_to_le32(new_snapids[1]);
 		n->v.subvol = 0;
 		SET_BCH_SNAPSHOT_SUBVOL(&n->v, false);
-		ret = bch2_trans_update(trans, &iter, &n->k_i, 0);
+		ret   = bch2_trans_update(trans, &iter, &n->k_i, 0);
 		if (ret)
 			goto err;
 	}