IB/iser: Fix missing return status check in iser_send_data_out

Since commit "IB/iser: Fix race between iser connection teardown..."
iser_initialize_task_headers() might fail, so we need to check that.

Fixes: 7414dde0

 (IB/iser: Fix race between iser connection ...)
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>