hpfs is the last user of generic_block_fiemap, so add a trivial
iomap_ops based on the ext2 version and switch to iomap_fiemap.

Link: https://lore.kernel.org/r/20210720133341.405438-4-hch@lst.deSigned-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>