Remove the hmm_mirror object and use the mmu_interval_notifier API instead
for the range, and use the normal mmu_notifier API for the general
invalidation callback.

While here re-organize the pagefault path so the locking pattern is clear.

nouveau is the only driver that uses a temporary range object and instead
forwards nearly every invalidation range directly to the HW. While this is
not how the mmu_interval_notifier was intended to be used, the overheads on
the pagefaulting path are similar to the existing hmm_mirror version.
Particularly since the interval tree will be small.

Link: https://lore.kernel.org/r/20191112202231.3856-10-jgg@ziepe.caTested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>