cxgbei: Better handle new tasks and transfers when disconnecting.

If the connection is in the process of disconnecting, ic_socket can be
NULL. For icl_cxgbei_conn_transfer_setup(), lock the connection and
check ic_socket before using it. For icl_cxgbei_conn_task_setup(),
the caller already holds the connection lock, so assert it and bail
early with ECONNRESET if the connection is disconnecting.

Reported by: Jithesh Arakkan @ Chelsio
Fixes: f949967c8eb3 cxgbei: Fix a race between transfer setup and a peer reset.