mm: mmu_notifier fix for tlb_end_vma

The generic tlb_end_vma does not call invalidate_range mmu notifier, and
it resets resets the mmu_gather range, which means the notifier won't be
called on part of the range in case of an unmap that spans multiple
vmas.

ARM64 seems to be the only arch I could see that has notifiers and uses
the generic tlb_end_vma.  I have not actually tested it.

[ Catalin and Will point out that ARM64 currently only uses the
  notifiers for KVM, which doesn't use the ->invalidate_range()
  callback right now, so it's a bug, but one that happens to
  not affect them.  So not necessary for stable.  - Linus ]

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>