mm: Remove return value of zap_vma_ptes()

All callers of zap_vma_ptes() are not interested in the return value of
that function, so let's simplify its interface and drop the return
value.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>