Pull MD fixes from Song.
* 'md-next' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md:
md-cluster: fix wild pointer of unlock_all_bitmaps()
md/raid5-cache: clear MD_SB_CHANGE_PENDING before flushing stripes
md: fix deadlock causing by sysfs_notify
md: improve io stats accounting
md: raid0/linear: fix dereference before null check on pointer mddev