btrfs: Remove unused length var from scrub_handle_errored_block

Added in b5d67f64 ("Btrfs: change scrub to support big blocks") but
rendered redundant by be50a8dd

 ("Btrfs: Simplify
scrub_setup_recheck_block()'s argument").

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>