This fix will ensure all SB copies on the disk is zeroed
when the disk is intentionally removed. This helps to
better manage disks in the user land.

This version of patch also merges the Zach patch as below.

 btrfs: don't double brelse on device rm
Signed-off-by: NAnand Jain <anand.jain@oracle.com>
Signed-off-by: NZach Brown <zab@redhat.com>
Signed-off-by: NChris Mason <clm@fb.com>