btrfs: create qgroup earlier in snapshot creation

Pull creating the qgroup earlier in the snapshot. This allows simple
quotas qgroups to see all the metadata writes related to the snapshot
being created and to be born with the root node accounted.

Note this has an impact on transaction commit where the qgroup creation
can do a lot of work, allocate memory and take locks. The change is done
for correctness, potential performance issues will be fixed in the
future.

Signed-off-by: Boris Burkov <boris@bur.io>
[ add note ]
Signed-off-by: David Sterba <dsterba@suse.com>