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

Pull MD updates from Song:

"The only significant change here is a fix in back_log sysfs entry, by
 Guoqing Jiang."

* 'md-next' of https://git.kernel.org/pub/scm/linux/kernel/git/song/md:
  raid5-ppl: use swap() to make code cleaner
  md/bitmap: don't set max_write_behind if there is no write mostly device