maple_tree: set the node limit when creating a new root node

Set the node limit of the root node so that the last pivot of all nodes is
the node limit (if the node is not full).

This patch also fixes a bug in mas_rev_awalk().  Effectively, always
setting a maximum makes mas_logical_pivot() behave as mas_safe_pivot(). 
Without this fix, it is possible that very small tasks would fail to find
the correct gap.  Although this has not been observed with real tasks, it
has been reported to happen in m68k nommu running the maple tree tests.

Link: https://lkml.kernel.org/r/20230711035444.526-1-zhangpeng.00@bytedance.com
Link: https://lore.kernel.org/linux-mm/CAMuHMdV4T53fOw7VPoBgPR7fP6RYqf=CBhD_y_vOg53zZX_DnA@mail.gmail.com/
Link: https://lkml.kernel.org/r/20230711035444.526-2-zhangpeng.00@bytedance.com
Fixes: 54a611b6 ("Maple Tree: add new data structure")
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -3692,7 +3692,8 @@ static inline int mas_root_expand(struct ma_state *mas, void *entry)
 	mas->offset = slot;
 	pivots[slot] = mas->last;
 	if (mas->last != ULONG_MAX)
-		slot++;
+		pivots[++slot] = ULONG_MAX;
+
 	mas->depth = 1;
 	mas_set_height(mas);
 	ma_set_meta(node, maple_leaf_64, 0, slot);