mm: merge hmm_vma_do_fault into into hmm_vma_walk_hole_

There is no good reason for this split, as it just obsfucates the flow.

Link: https://lore.kernel.org/r/20200316135310.899364-6-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>