block: fix sleeping function called from invalid context in part_get_stat_info()

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I57S8D


CVE: NA

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part_get_stat_info() call mutex_lock() inside part_stat_lock(), which
is wrong because part_stat_lock() disables preempt.

Fix the problem by hold mutex first.

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>