mm, mmu_notifier: be explicit about range invalition non-blocking mode

If invalidate_range_start() is called for !blocking mode then all
callbacks have to guarantee they will no block/sleep.  The same obviously
applies to invalidate_range_end because this operation pairs with the
former and they are called from the same context.  Make sure this is
appropriately documented.

Link: http://lkml.kernel.org/r/20180827112623.8992-3-mhocko@kernel.org


Signed-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>