target: move depth_left manipulation out of transport_generic_request_failure

We only need to decrement dev->depth_left if failing a command from
__transport_execute_tasks.  Instead of doing it first thing in
transport_generic_request_failure and requiring a pseudo-flag argument
for it just opencode the decrement in the two callers (which should
be factored into a single one anyway)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>