ext4: move ext4 bmap to use iomap infrastructure

ext4_iomap_begin is already implemented which provides ext4_map_blocks,
so just move the API from generic_block_bmap to iomap_bmap for iomap
conversion.
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/8bbd53bd719d5ccfecafcce93f2bf1d7955a44af.1582880246.git.riteshh@linux.ibm.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/inode.c

@@ -3212,7 +3212,7 @@ static sector_t ext4_bmap(struct address_space *mapping, sector_t block)
 			return 0;
 	}
 
-	return generic_block_bmap(mapping, block, ext4_get_block);
+	return iomap_bmap(mapping, block, &ext4_iomap_ops);
 }
 
 static int ext4_readpage(struct file *file, struct page *page)