CVE-2023-53421: 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 blkgiostatset after clearing in blkcgresetstats()

When blkgalloc() is called to allocate a blkcggq structure with the associated blkgiostatset's, there are 2 fields within blkgiostatset that requires proper initialization - blkg & sync. The former field was introduced by commit 3b8cc6298724 ("blk-cgroup: Optimize blkcgrstatflush()") while the later one was introduced by commit f73316482977 ("blk-cgroup: reimplement basic IO stats using cgroup rstat").

Unfortunately those fields in the blkgiostatset's are not properly re-initialized when they are cleared in v1's blkcgresetstats(). 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.