f2fs: enable f2fs_migrate_page

When f2fs introduces f2fs_migrate_page, we disabled it for 3.18 due to not much
validation. I couldn't find any isse on this feature in v3.18 when testing
xfstests/fsstress for a while. Let's enable it.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>