mm/rmap: split migration into its own function

Migration is currently implemented as a mode of operation for
try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag
or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE.

However it does not have much in common with the rest of the unmap
functionality of try_to_unmap_one() and thus splitting it into a separate
function reduces the complexity of try_to_unmap_one() making it more
readable.

Several simplifications can also be made in try_to_migrate_one() based on
the following observations:

 - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK.
 - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON.
 - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH.

TTU_SPLIT_FREEZE is a special case of migration used when splitting an
anonymous page.  This is most easily dealt with by calling the correct
function from unmap_page() in mm/huge_memory.c - either try_to_migrate()
for PageAnon or try_to_unmap().

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alistair Popple <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Ralph Campbell <[email protected]>
Cc: Ben Skeggs <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: "Matthew Wilcox (Oracle)" <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Shakeel Butt <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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