ext2: use iomap_fiemap to implement ->fiemap

Switch from generic_block_fiemap to use the iomap version.  The only
interesting part is that ext2_get_blocks gets confused when being
asked for overly long ranges, so copy over the limit to the inode
size from generic_block_fiemap into ext2_fiemap.

Link: https://lore.kernel.org/r/20210720133341.405438-3-hch@lst.de


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