mm: Export migrate_page_move_mapping and migrate_page_copy

commit 1118dce773d84f39ebd51a9fe7261f9169cb056e upstream.

Export these symbols such that UBIFS can implement
->migratepage.

Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Christoph Hellwig <hch@lst.de>
[bwh: Backported to 3.2: also change migrate_page_move_mapping() from
 static to extern, done as part of an earlier commit upstream]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>