Replace a lot of formulaic multiplications (containing casts, no less)
with calls to a pair of functions. This encapsulates in a single
place the operations which require care relating to integer overflow.
Cc: Dongxiao Xu <dongxiao.xu@intel.com> Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com> Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>