mm/mmu_notifiers: prime lockdep

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/[email protected]
Suggested-by: Jason Gunthorpe <[email protected]>
Reviewed-by: Jason Gunthorpe <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Jason Gunthorpe <[email protected]>