mm: migrate: clean up migrate_prep{_local}

mm: migrate: clean up migrate_prep{_local}

The migrate_prep{_local} never fails, so it is pointless to have return
value and check the return value.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yang Shi <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Cc: Jan Kara <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Mel Gorman <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Song Liu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>