daemon: chroot: Fix long-standing possible deadlock.

The child (chrooted) process wrote its answer on the pipe and then
exited.  Meanwhile the parent waiting for the child to exit before
reading from the pipe.  Thus if the output was larger than a Linux
pipebuffer then the whole thing would deadlock.

daemon/chroot.ml

@@ -62,6 +62,10 @@ let f t func arg =
   (* Parent. *)
   close wfd;
 
+  let chan = in_channel_of_descr rfd in
+  let ret = input_value chan in
+  close_in chan;
+
   let _, status = waitpid [] pid in
   (match status with
    | WEXITED 0 -> ()
@@ -76,10 +80,6 @@ let f t func arg =
       failwithf "chroot ‘%s’ stopped by signal %d" t.name i
   );
 
-  let chan = in_channel_of_descr rfd in
-  let ret = input_value chan in
-  close_in chan;
-
   match ret with
   | Either ret -> ret
   | Or exn -> raise exn