mm: check for truncation in vmalloc_type()

mm: check for truncation in vmalloc_type()

While it's currently implied from the checking xmalloc_array() does,
let's make this more explicit in the function itself. As a result both
involved local variables don't need to have size_t type anymore. This
brings them in line with the rest of the code in this file.

Requested-by: Julien Grall <julien@xen.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <jgrall@amazon.com>