In start_transactio(), we will try to join the transaction again after
the current transaction is committed, so we should not release the
reserved space of the qgroup. Fix it.

Cc: Arne Jansen <sensille@gmx.net>
Signed-off-by: NMiao Xie <miaox@cn.fujitsu.com>
Signed-off-by: NJosef Bacik <jbacik@fusionio.com>