mptfusion: prevent some memory corruption

These are signed values the come from the user, we put a cap on the
upper bounds but not on the lower bounds.

We use "karg.dataSgeOffset" to calculate "sz".  We verify "sz" and
proceed as if that means that "karg.dataSgeOffset" is correct but this
fails to consider that the "sz" calculations can have integer overflows.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: James Bottomley <JBottomley@Odin.com>