mm/hmm: simplify hmm_vma_walk_hugetlb_entry()

Remove the rather confusing goto label and just handle the fault case
directly in the branch checking for it.

Link: https://lore.kernel.org/r/20200316135310.899364-4-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>