blk-wbt: fix possible deadlock to nest pcpu_alloc_mutex under q_usage_counter

blk-wbt: fix possible deadlock to nest pcpu_alloc_mutex under q_usage_counter

If wbt is disabled by default and user configures wbt by sysfs, queue
will be frozen first and then pcpu_alloc_mutex will be held in
blk_stat_alloc_callback().

Fix this problem by allocating memory first before queue frozen.

Signed-off-by: Yu Kuai <yukuai@fnnas.com>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>