hmm_range_fault() calls find_vma() and walk_page_range() in a loop.  This
is unnecessary duplication since walk_page_range() calls find_vma() in a
loop already.

Simplify hmm_range_fault() by defining a walk_test() callback function to
filter unhandled vmas.

This also fixes a bug where hmm_range_fault() was not checking start >=
vma->vm_start before checking vma->vm_flags so hmm_range_fault() could
return an error based on the wrong vma for the requested range.

It also fixes a bug when the vma has no read access and the caller did not
request a fault, there shouldn't be any error return code.

Link: https://lore.kernel.org/r/20191104222141.5173-2-rcampbell@nvidia.comSigned-off-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Christoph Hellwig <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>