docs: deprecated.rst: Clarify open-coded arithmetic with literals

docs: deprecated.rst: Clarify open-coded arithmetic with literals

Although using literals for size calculation in allocator arguments may
be harmless due to compiler warnings in case of overflows, it is better
to refactor the code to avoid the use of open-coded arithmetic.

So, clarify the preferred way in these cases.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Len Baker <len.baker@gmx.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20210925143455.21221-1-len.baker@gmx.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>