This fixes the instances of the "Multiplication result converted to
larger type" alert that codeQL scanning found.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Andrew Innes <andrew.c12@gmail.com>
Closes#14094