mm/migrate.c: cleanup expected_page_refs()

mm/migrate.c: cleanup expected_page_refs()

Andrea has noted that page migration code propagates page_mapping(page)
through the whole migration stack down to migrate_page() function so it
seems stupid to then use page_mapping(page) in expected_page_refs()
instead of passed down 'mapping' argument.  I agree so let's make
expected_page_refs() more in line with the rest of the migration stack.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Jan Kara <[email protected]>
Suggested-by: Andrea Arcangeli <[email protected]>
Reviewed-by: Andrea Arcangeli <[email protected]>
Cc: Mel Gorman <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>