btrfs: remove redundant variables from __process_folios_contig() and lock_delalloc_folios()

Same pattern in both functions, we really only use index, start_index is
redundant.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>