rds:Remove unnecessary ib_ring unalloc

In the function rds_ib_xmit_atomic, ib_ring is not allocated
successfully. As such, it is not necessary to unalloc it.

Cc: Joe Jin <joe.jin@oracle.com>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>