mm/hmm: invalidate device page table at start of invalidation

Invalidate device page table at start of invalidation and invalidate in
progress CPU page table snapshooting at both start and end of any
invalidation.

This is helpful when device need to dirty page because the device page
table report the page as dirty.  Dirtying page must happen in the start
mmu notifier callback and not in the end one.

Link: http://lkml.kernel.org/r/20181019160442.18723-7-jglisse@redhat.com


Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Cc: Ralph Campbell <rcampbell@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>