btrfs: document modified parameter of add_extent_mapping

Fixes fs/btrfs/extent_map.c:399: warning: Function parameter or member
'modified' not described in 'add_extent_mapping'

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>