Merge tag 'md/3.12-fixes' of git://neil.brown.name/md

Pull md bugfixes from Neil Brown:
 "Assorted md bug-fixes for 3.12.

  All tagged for -stable releases too"

* tag 'md/3.12-fixes' of git://neil.brown.name/md:
  raid5: avoid finding "discard" stripe
  raid5: set bio bi_vcnt 0 for discard request
  md: avoid deadlock when md_set_badblocks.
  md: Fix skipping recovery for read-only arrays.