IB/iser: Fix a possible race in iser connection states transition

In some circumstances (multiple targets), RDMA_CM ESTABLISHED event
and ep_disconnect may race. In this case, the iser connection state
may transition to UP (after ep_disconnect transitioned it to
TERMINATING), while the connection is being torn down.

Upon RDMA_CM event ESTABLISHED we allow iser connection state to
transition to UP only from PENDING. We also make sure to protect this
state change (done under the connection lock).

Signed-off-by: Ariel Nahum <arieln@mellanox.com>
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>