f2fs: Correct f2fs_dirty_data_folio() conversion

I got the return value wrong.  Very little checks the return value
from set_page_dirty(), so nobody noticed during testing.

Fixes: 4f5e34f7 ("f2fs: Convert f2fs_set_data_page_dirty to f2fs_dirty_data_folio")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Al Viro <viro@zeniv.linux.org.uk>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>