bcachefs: Fix blocking with locks held

This is a major oopsy - we should always be unlocking before calling
closure_sync(), else we'll cause a deadlock.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update_interior.c

@@ -1117,10 +1117,10 @@ bch2_btree_update_start(struct btree_trans *trans, struct btree_path *path,
 
 		closure_init_stack(&cl);
 
-		bch2_trans_unlock(trans);
-
 		do {
 			ret = bch2_btree_reserve_get(trans, as, nr_nodes, flags, &cl);
+
+			bch2_trans_unlock(trans);
 			closure_sync(&cl);
 		} while (ret == -EAGAIN);
 	}