btrfs: don't take an extra root ref at allocation time

Now that all the users of roots take references for them we can drop the
extra root ref we've been taking.  Before we had roots at 2 refs for the
life of the file system, one for the radix tree, and one simply for
existing.  Now that we have proper ref accounting in all places that use
roots we can drop this extra ref simply for existing as we no longer
need it.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>