gpu: nova-core: use checked arithmetic in Booter signature parsing

gpu: nova-core: use checked arithmetic in Booter signature parsing

Use checked_add() when computing signature offsets from firmware-
provided values in signatures_iter().

Without checked arithmetic, overflow could wrap to a small plausible
offset that points to entirely wrong data.

Reviewed-by: Zhi Wang <zhiw@nvidia.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260126202305.2526618-3-joelagnelf@nvidia.com
[acourbot@nvidia.com: remove obvious computation comments.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>