IB/srp: Fix a connection setup race

Avoid that receiving a DREQ while RDMA channels are being
established causes target->qp_in_error to be reset.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Sagi Grimberg <sagig@mellanox.com>
Cc: Sebastian Parschauer <sebastian.riemer@profitbricks.com>
Cc: <stable@vger.kernel.org> #v3.19
Signed-off-by: Doug Ledford <dledford@redhat.com>