mm/migrate_device.c: refactor migrate_vma and migrate_deivce_coherent_page()

mm/migrate_device.c: refactor migrate_vma and migrate_deivce_coherent_page()

migrate_device_coherent_page() reuses the existing migrate_vma family of
functions to migrate a specific page without providing a valid mapping or
vma.  This looks a bit odd because it means we are calling migrate_vma_*()
without setting a valid vma, however it was considered acceptable at the
time because the details were internal to migrate_device.c and there was
only a single user.

One of the reasons the details could be kept internal was that this was
strictly for migrating device coherent memory.  Such memory can be copied
directly by the CPU without intervention from a driver.  However this
isn't true for device private memory, and a future change requires similar
functionality for device private memory.  So refactor the code into
something more sensible for migrating device memory without a vma.

Link: https://lkml.kernel.org/r/c7b2ff84e9b33d022cf4a40f87d051f281a16d8f.1664366292.git-series.apopple@nvidia.com
Signed-off-by: Alistair Popple <[email protected]>
Cc: "Huang, Ying" <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Ralph Campbell <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Alex Sierra <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Christian König <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Felix Kuehling <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Michael Ellerman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>