dm btree: add a defensive bounds check to insert_at()

stable inclusion
from linux-4.19.226
commit 716e490c87e4421d5908f7818fc6b61536a17744

--------------------------------

[ Upstream commit 85bca3c0 ]

Corrupt metadata could trigger an out of bounds write.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>