Merge branch 'md-next' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md...

Pull MD updates from Song:

"1. mddev_find_or_alloc() clean up, from Christoph.
 2. Fix NULL pointer deref with external bitmap, from Sudhakar."

* 'md-next' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md:
  md/bitmap: wait for external bitmap writes to complete during tear down
  md: do not return existing mddevs from mddev_find_or_alloc
  md: refactor mddev_find_or_alloc
  md: factor out a mddev_alloc_unit helper from mddev_find