mm/migrate.c: remove MIGRATE_PFN_LOCKED

mm/migrate.c: remove MIGRATE_PFN_LOCKED

MIGRATE_PFN_LOCKED is used to indicate to migrate_vma_prepare() that a
source page was already locked during migrate_vma_collect().  If it
wasn't then the a second attempt is made to lock the page.  However if
the first attempt failed it's unlikely a second attempt will succeed,
and the retry adds complexity.  So clean this up by removing the retry
and MIGRATE_PFN_LOCKED flag.

Destination pages are also meant to have the MIGRATE_PFN_LOCKED flag
set, but nothing actually checks that.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Alistair Popple <[email protected]>
Reviewed-by: Ralph Campbell <[email protected]>
Acked-by: Felix Kuehling <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Jerome Glisse <[email protected]>
Cc: John Hubbard <[email protected]>
Cc: Zi Yan <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Ben Skeggs <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>