[RFC] MMU Notifiers V1

This is a patchset implementing MMU notifier callbacks based on Andrea's
earlier work. These are needed if Linux pages are referenced from something
else than tracked by the rmaps of the kernel.

To do:

- Make locking requirements for the callbacks consistent and
  document them accurately.
- Insure that the callbacks are complete
- Feedback from uses of the callbacks for KVM, RDMA, XPmem and GRU

Andrea's mmu_notifier #4 -> RFC V1

- Merge subsystem rmap based with Linux rmap based approach
- Move Linux rmap based notifiers out of macro
- Try to account for what locks are held while the notifiers are
  called.
- Develop a patch sequence that separates out the different types of
  hooks so that it is easier to review their use.
- Avoid adding #include to linux/mm_types.h
- Integrate RCU logic suggested by Peter.

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