We want to see if the requested start or total block count are
unaligned.  We discard the whole numbers and only care about the
remainder.  Update the code to use div_u64_rem here and add a comment.

Cc: Hans de Goede <hdegoede@redhat.com>
Cc: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Cc: Bernhard Nortmann <bernhard.nortmann@web.de>
Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
Tested-by: Bernhard Nortmann <bernhard.nortmann@web.de>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>