maple_tree: fix mas_spanning_rebalance() on insufficient data

Mike Rapoport contacted me off-list with a regression in running criu. 
Periodic tests fail with an RCU stall during execution.  Although rare, it
is possible to hit this with other uses so this patch should be backported
to fix the regression.

This patchset adds the fix and a test case to the maple tree test
suite.


This patch (of 2):

An insufficient node was causing an out-of-bounds access on the node in
mas_leaf_max_gap().  The cause was the faulty detection of the new node
being a root node when overwriting many entries at the end of the tree.

Fix the detection of a new root and ensure there is sufficient data prior
to entering the spanning rebalance loop.

Link: https://lkml.kernel.org/r/20221219161922.2708732-1-Liam.Howlett@oracle.com
Link: https://lkml.kernel.org/r/20221219161922.2708732-2-Liam.Howlett@oracle.com
Fixes: 54a611b6

 ("Maple Tree: add new data structure")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Mike Rapoport <rppt@kernel.org>
Tested-by: Mike Rapoport <rppt@kernel.org>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>