Since the allocated memory is going to be zeroed immediately anyway,
let's avoid redundantly scrubbing it with MALLOC_SCRUB_BYTE just before
that.
The latest versions of gcc and Clang can automatically do this malloc +
memset -> calloc optimization, but I've seen a couple of places where it
failed to be done.
This commit also adds a naive kcalloc function to the kernel that
doesn't (yet) eliminate the redundancy like the userland does.
e3eb68dd58