This converts one of the two users of mmu_notifiers to use the new API.
The conversion is fairly straightforward, however the existing use of
notifiers here seems to be racey.

Link: https://lore.kernel.org/r/20191112202231.3856-7-jgg@ziepe.caTested-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>