btrfs: check ticket->steal in steal_from_global_block_rsv

We're going to use this helper in the priority flushing loop, move this
check into the helper to simplify the logic.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>