maple_tree: add smp_rmb() to dead node detection

commit 0a2b18d9 upstream.

Add an smp_rmb() before reading the parent pointer to ensure that anything
read from the node prior to the parent pointer hasn't been reordered ahead
of this check.

The is necessary for RCU mode.

Link: https://lkml.kernel.org/r/20230227173632.3292573-7-surenb@google.com
Fixes: 54a611b6

 ("Maple Tree: add new data structure")
Cc: stable@vger.kernel.org
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>