Replace the internal interval tree based mmu notifier with the new common
mmu_interval_notifier_insert() API. This removes a lot of code and fixes a
deadlock that can be triggered in ODP:

 zap_page_range()
  mmu_notifier_invalidate_range_start()
   [..]
    ib_umem_notifier_invalidate_range_start()
       down_read(&per_mm->umem_rwsem)
  unmap_single_vma()
    [..]
      __split_huge_page_pmd()
        mmu_notifier_invalidate_range_start()
        [..]
           ib_umem_notifier_invalidate_range_start()
              down_read(&per_mm->umem_rwsem)   // DEADLOCK

        mmu_notifier_invalidate_range_end()
           up_read(&per_mm->umem_rwsem)
  mmu_notifier_invalidate_range_end()
     up_read(&per_mm->umem_rwsem)

The umem_rwsem is held across the range_start/end as the ODP algorithm for
invalidate_range_end cannot tolerate changes to the interval
tree. However, due to the nested invalidation regions the second
down_read() can deadlock if there are competing writers. The new core code
provides an alternative scheme to solve this problem.

Fixes: ca748c39 ("RDMA/umem: Get rid of per_mm->notifier_count")
Link: https://lore.kernel.org/r/20191112202231.3856-6-jgg@ziepe.caTested-by: NArtemy Kovalyov <artemyko@mellanox.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>