UPSTREAM: 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.

Bug: 180018981
Link: https://lkml.kernel.org/r/20201113205359.556831-5-shy828301@gmail.com


Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 236c32eb)
Signed-off-by: Minchan Kim <minchan@google.com>
Change-Id: I26ae0a4240c704e6127b51532e5de8ba778edd6e