mm: vmalloc: introduce array allocation functions

Linux has dozens of occurrences of vmalloc(array_size()) and
vzalloc(array_size()).  Allow to simplify the code by providing
vmalloc_array and vcalloc, as well as the underscored variants that let
the caller specify the GFP flags.

Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>