IB/srp: Fix memory leak on reconnect

SRP reallocates the IU buffers for tx_ring and rx_ring without freeing
the old buffers when it reconnects to a target.  Fix this by keeping
the old IU buffers around.

Signed-off-by: Vu Pham <vu@mellanox.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>