cxgbei: Don't fail task setup if the socket is disconnected.

When the initiator is reconnecting to the target, the connection may
temporarily be marked disconnected or not have an associated socket.
New I/O requests received by the initiator in this state should not
fail with ECONNRESET as that results in an I/O error back to userland.
Instead, they need to still succeed so that CAM can queue the requests
and send them once the connection is re-established.

Setting up DDP for zero-copy receive requires a socket, so just punt
on using DDP for these transfers.

Reported by: Jithesh Arakkan @ Chelsio
Sponsored by: Chelsio Communications

(cherry picked from commit 752e211e64699ff14bb0a66d368cfaec836cfb95)