block: fix order error in blk_release_queue

hulk inclusion
category: bugfix, https://gitee.com/openeuler/kernel/issues/I64SY5


bugzilla: 188093
CVE: NA

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blk_stat_free_callback() will check stats->callbacks, and wbt's callback
is freed in wbt_exit(), which is called by rq_qos_exit(). So move it in
front of blk_stat_free_callback().

Fixes: 015466fc61f4 ("block: fix null-pointer dereference in ioc_pd_init")
Signed-off-by: Li Nan <linan122@huawei.com>