blk-cgroup: Reinit blkg_iostat_set after clearing in blkcg_reset_stats()

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

blk-cgroup: Reinit blkg_iostat_set after clearing in blkcg_reset_stats()

When blkg_alloc() is called to allocate a blkcg_gq structure

with the associated blkg_iostat_set's, there are 2 fields within

blkg_iostat_set that requires proper initialization - blkg & sync.

The former field was introduced by commit 3b8cc6298724 ("blk-cgroup:

Optimize blkcg_rstat_flush()") while the later one was introduced by

commit f73316482977 ("blk-cgroup: reimplement basic IO stats using

cgroup rstat").

Unfortunately those fields in the blkg_iostat_set's are not properly

re-initialized when they are cleared in v1's blkcg_reset_stats(). This

can lead to a kernel panic due to NULL pointer access of the blkg

pointer. The missing initialization of sync is less problematic and

can be a problem in a debug kernel due to missing lockdep initialization.

Fix these problems by re-initializing them after memory clearing.