IB/iser: Micro-optimize iser_handle_wc

Use likely() for wc.status == IB_WC_SUCCESS

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