mm/mmu_notifiers: prime lockdep

We want to teach lockdep that mmu notifiers can be called from direct
reclaim paths, since on many CI systems load might never reach that
level (e.g. when just running fuzzer or small functional tests).

I've put the annotation into mmu_notifier_register since only when we have
mmu notifiers registered is there any point in teaching lockdep about
them. Also, we already have a kmalloc(, GFP_KERNEL), so this is safe.

Link: https://lore.kernel.org/r/20190826201425.17547-3-daniel.vetter@ffwll.ch
Suggested-by: Jason Gunthorpe <jgg@mellanox.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>