Btrfs: make subvolid=0 mount the original default root

Since theres not a good way to make sure the user sees the original default root
tree id, and not to mention it's 5 so is way different than any other volume,
just make subvol=0 mount the original default root.  This makes it a bit easier
for users to handle in the long run.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>