Commit 4d4bbd85 ("mm, oom_reaper: skip mm structs with mmu
notifiers") prevented the oom reaper from unmapping private anonymous
memory with the oom reaper when the oom victim mm had mmu notifiers
registered.

The rationale is that doing mmu_notifier_invalidate_range_{start,end}()
around the unmap_page_range(), which is needed, can block and the oom
killer will stall forever waiting for the victim to exit, which may not
be possible without reaping.

That concern is real, but only true for mmu notifiers that have
blockable invalidate_range_{start,end}() callbacks.  This patch adds a
"flags" field to mmu notifier ops that can set a bit to indicate that
these callbacks do not block.

The implementation is steered toward an expensive slowpath, such as
after the oom reaper has grabbed mm->mmap_sem of a still alive oom
victim.

[rientjes@google.com: mmu_notifier_invalidate_range_end() can also call the invalidate_range() must not b...