[bug] mkfs.btrfs reports device busy for ext4 mounted disk

[Anand Jain <Anand.Jain@oracle.com>]

setup:
  mkfs.btrfs /dev/sdb
  mkfs.ext4 /dev/sdb && mount /dev/sdb /ext4
  mkfs.btrfs /dev/sdc /dev/sdd


test case:
  mkfs.btrfs /dev/sdc /dev/sdd


problem:
  mkfs is fine, however reports the following error ..
  ---
   ERROR: unable to scan the device '/dev/sdb' - Device or resource busy
   ERROR: unable to scan the device '/dev/sdb' - Device or resource busy
  ---


findings:
  First, since previously we have had multi-device btrfs,
  so mkfs.btrfs would trigger scan for its partner to check
  if its mounted.

  Next, since previously we had btrfs on sdb and mkfs.ext4
  does not overwrite super-block mirror 1.. so

    btrfs_read_dev_super(int fd, struct btrfs_super_block *sb, u64 
sb_bytenr)

  finds btrfs on sdb. So we try to register sdb with the ioctl
  BTRFS_IOC_SCAN_DEV and fail.


  unless I am missing something. wipefs (along with the below patch)
     [PATCH][v2] Btrfs: wipe all the superblock [redhat bugzilla 889888]
  seems to be only solution as of now.

  Any idea if there can be a better solution to handle stale btrfs
  and can be done with in the btrfs limits.

Thanks, Anand