gpu: nova-core: use checked arithmetic in FWSEC firmware parsing

gpu: nova-core: use checked arithmetic in FWSEC firmware parsing

Use checked_add() and checked_mul() when computing offsets from

firmware-provided values in new_fwsec().
Without checked arithmetic, corrupt firmware could cause integer
overflow. The danger is not just wrapping to a huge value, but
potentially wrapping to a small plausible offset that passes validation
yet accesses entirely wrong data, causing silent corruption or security
issues.

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/20260126202305.2526618-2-joelagnelf@nvidia.com
[acourbot@nvidia.com: rewrap commit message to make checkpatch happy.]
[acourbot@nvidia.com: add missing empty lines after new code blocks.]
[acourbot@nvidia.com: move SAFETY comments to the unsafe statement they
 describe.]
[acourbot@nvidia.com: remove obvious computation comments and use
`CALC:` for the remaining ones.]
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>