overflow: Make sure size helpers are always inlined

overflow: Make sure size helpers are always inlined

With kmalloc_obj() performing implicit size calculations, the embedded
size_mul() calls, while marked inline, were not always being inlined.
I noticed a couple places where allocations were making a call out for
things that would otherwise be compile-time calculated. Force the
compilers to always inline these calculations.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://patch.msgid.link/20260224232451.work.614-kees@kernel.org
Signed-off-by: Kees Cook <kees@kernel.org>