FROMLIST: mm: add mmu_notifier_lock

Introduce mmu_notifier_lock as a per-mm percpu_rw_semaphore,
as well as the code to initialize and destroy it together with the mm.

This lock will be used to prevent races between mmu_notifier_register()
and speculative fault handlers that need to fire MMU notifications
without holding any of the mmap or rmap locks.

Signed-off-by: Michel Lespinasse <michel@lespinasse.org>
Link: https://lore.kernel.org/all/20220128131006.67712-24-michel@lespinasse.org/


Bug: 161210518
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I453ebe979c8b9dcc6159b41c5ec7a1ea17d85ee2