Fix calloc(3) arguments order

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