lib/mpi: added comment on divide by 0 case

Comment explains that existing clients do not call this function
with dsize == 0, which means that 1/0 should not happen.

Signed-off-by: Dmitry Kasatkin <dmitry.kasatkin@intel.com>
Reviewed-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: James Morris <jmorris@namei.org>