mm/mmu_notifier: Rename struct mmu_notifier_mm to mmu_notifier_subscriptions

The name mmu_notifier_mm implies that the thing is a mm_struct pointer,
and is difficult to abbreviate. The struct is actually holding the
interval tree and hlist containing the notifiers subscribed to a mm.

Use 'subscriptions' as the variable name for this struct instead of the
really terrible and misleading 'mmn_mm'.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>