The pagewalker does not call most ops with NULL vma, those are all routed
to hmm_vma_walk_hole() via ops->pte_hole instead.

Thus hmm_vma_fault() is only called with a NULL vma from
hmm_vma_walk_hole(), so hoist the NULL vma check to there.

Now it is clear that snapshotting with no vma is a HMM_PFN_ERROR as
without a vma we have no path to call hmm_vma_fault().

Link: https://lore.kernel.org/r/20200327200021.29372-10-jgg@ziepe.caReviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>