Replace zed's use of malloc with calloc

When zed allocates memory via malloc(), it typically follows that
with a memset().  However, calloc() implementations can often perform
optimizations when zeroing memory:

https://stackoverflow.com/questions/2688466/why-mallocmemset-is-slower-than-calloc

This commit replaces zed's use of malloc() with calloc().

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2736