maple_tree: be more cautious about dead nodes

maple_tree: be more cautious about dead nodes

ma_pivots() and ma_data_end() may be called with a dead node.  Ensure to
that the node isn't dead before using the returned values.

This is necessary for RCU mode of the maple tree.

Link: https://lkml.kernel.org/r/20230227173632.3292573-1-surenb@google.com
Link: https://lkml.kernel.org/r/20230227173632.3292573-2-surenb@google.com
Fixes: 54a611b60590 ("Maple Tree: add new data structure")
Cc: <Stable@vger.kernel.org>
Signed-off-by: Liam Howlett <Liam.Howlett@oracle.com>