mm: export buffer_migrate_folio_norefs()

Ext4 needs this function to allow safe migration for journalled data
pages.

Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20221207112722.22220-11-jack@suse.cz


Signed-off-by: Theodore Ts'o <tytso@mit.edu>