From: Hugh Dickins <hugh@veritas.com>

We're NULLifying more and more fields when initializing a vma
(mpol_set_vma_default does that too, if configured to do anything).  Now use
memset to avoid specifying fields, and save a little code too.

(Yes, I realize anon_vma will want to set vm_pgoff non-0, but I think that
will be better handled at the core, since anon vm_pgoff is negotiable up until
an anon_vma is actually assigned.)