btrfs: simplify parameters of btrfs_set_disk_extent_flags

All callers pass extent buffer start and length so the extent buffer
itself should work fine.

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