maple_tree: make mas_validate_limits() check root node and node limit

Update mas_validate_limits() to check root node, check node limit pivot if
there is enough room for it to exist and check data_end.  Remove the check
for child existence as it is done in mas_validate_child_slot().

Link: https://lkml.kernel.org/r/20230711035444.526-6-zhangpeng.00@bytedance.com


Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>