mm/mmu_notifiers: hoist do_mmu_notifier_register down_write to the caller

This simplifies the code to not have so many one line functions and extra
logic. __mmu_notifier_register() simply becomes the entry point to
register the notifier, and the other one calls it under lock.

Also add a lockdep_assert to check that the callers are holding the lock
as expected.

Link: https://lore.kernel.org/r/20190806231548.25242-2-jgg@ziepe.ca
Suggested-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>