ext2: make ext2_iomap_ops available unconditionally

ext2_iomap_ops will be used for the FIEMAP support going forward,
so make it available unconditionally.

Link: https://lore.kernel.org/r/20210720133341.405438-2-hch@lst.de


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>