Online device removal pushed to the unstable trees

Hello everyone,

I've pushed out some code to remove devices online. There are no disk format changes included. The basic usage is to:

mkfs.btrfs /dev/sdb /dev/sdc /dev/sdd /dev/sde
# mount, do things

btrfs-vol -r /dev/sdc /mnt

Any blocks on /dev/sdc will be relocated to other chunks. The algorithm is very simple but it works for all the raid levels. A future project will be to optimize this code for drive replacement.

There are also fixes in there for the code that does mount -o subvol=<name>

Next on my list is the ability to mount a degraded filesystem.

-chris