block: Fix kabi broken in blk-merge.h and blk-cgroup.h

hulk inclusion
category: bugfix
bugzilla: 187443, https://gitee.com/openeuler/kernel/issues/I5Z7O2


CVE: NA

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Include additional files and add new function will cause kabi broken. So
move changes to blk-mq.h. bio_issue_as_root_blkg() is needed by
blk_cgroup_mergeable(), move it together. It is used by iocost, too, so
add blk-mq.h to blk-iocost.c.

Signed-off-by: Li Nan <linan122@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>