test_maple_tree: test modifications while iterating

[ Upstream commit 5159d64b ]

Add a testcase to ensure the iterator detects bad states on modifications
and does what the user expects

Link: https://lkml.kernel.org/r/20230120162650.984577-5-Liam.Howlett@oracle.com


Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Stable-dep-of: 7a93c71a ("maple_tree: fix 32 bit mas_next testing")
Signed-off-by: Sasha Levin <sashal@kernel.org>