Fix calloc(3) arguments order

calloc(3) takes nelem (or nmemb in glibc) first, and then size of
elements. No difference expected for having these in reverse order,
however should follow the standard.

http://pubs.opengroup.org/onlinepubs/009695399/functions/calloc.html

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@osnexus.com>
Closes #7405