btrfs: bail out earlier from need_preemptive_reclaim() if we have tickets

btrfs: bail out earlier from need_preemptive_reclaim() if we have tickets

Instead of doing some calculations and then return false if it turns out
we have queued tickets, check first if we have tickets and return false
immediately if we have tickets, without wasting time on doing those
computations.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>