IB/srp: Avoid sending a task management function needlessly

Do not send a task management function if sending will fail anyway
because either there is no RDMA/RC connection or the QP is in the
error state.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: David Dillow <dave@thedillows.org>
Cc: <stable@vger.kernel.org> # 3.8
Signed-off-by: Roland Dreier <roland@purestorage.com>