cxgb4: Add missing annotation for service_ofldq()

Sparse reports a warning at service_ofldq()

warning: context imbalance in service_ofldq() - unexpected unlock

The root cause is the missing annotation at service_ofldq()

Add the missing __must_hold(&q->sendq.lock) annotation

Signed-off-by: Jules Irenge <jbi.octave@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>