blk-iolatency: Fix memory leak on add_disk() failures

In the Linux kernel, the following vulnerability has been resolved:

blk-iolatency: Fix memory leak on add_disk() failures

When a gendisk is successfully initialized but add_disk() fails such as when

a loop device has invalid number of minor device numbers specified,

blkcg_init_disk() is called during init and then blkcg_exit_disk() during

error handling. Unfortunately, iolatency gets initialized in the former but

doesn't get cleaned up in the latter.

This is because, in non-error cases, the cleanup is performed by

del_gendisk() calling rq_qos_exit(), the assumption being that rq_qos

policies, iolatency being one of them, can only be activated once the disk

is fully registered and visible. That assumption is true for wbt and iocost,

but not so for iolatency as it gets initialized before add_disk() is called.

It is desirable to lazy-init rq_qos policies because they are optional

features and add to hot path overhead once initialized - each IO has to walk

all the registered rq_qos policies. So, we want to switch iolatency to lazy

init too. However, that's a bigger change. As a fix for the immediate

problem, let's just add an extra call to rq_qos_exit() in blkcg_exit_disk().

This is safe because duplicate calls to rq_qos_exit() become noop's.