mm/migrate: Add filemap_migrate_folio()

There is nothing iomap-specific about iomap_migratepage(), and it fits
a pattern used by several other filesystems, so move it to mm/migrate.c,
convert it to be filemap_migrate_folio() and convert the iomap filesystems
to use it.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Darrick J. Wong <[email protected]>
7 files changed