migration: only migrate_prep() once per move_pages()

migrate_prep() is fairly expensive (72us on 16-core barcelona 1.9GHz).
Commit 3140a2273009c01c27d316f35ab76a37e105fdd8 improved move_pages()
throughput by breaking it into chunks, but it also made migrate_prep() be
called once per chunk (every 128pages or so) instead of once per
move_pages().

This patch reverts to calling migrate_prep() only once per chunk as we did
before 2.6.29.  It is also a followup to commit
0aedadf91a70a11c4a3e7c7d99b21e5528af8d5d ("mm: move migrate_prep out from
under mmap_sem").

This improves migration throughput on the above machine from 600MB/s to
750MB/s.

Signed-off-by: Brice Goglin <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Rik van Riel <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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