nvmet-fcloop: drop response if targetport is gone

When the target port is gone, the lsrsp pointer is invalid. Thus don't
call the done function anymore instead just drop the response.

This happens when the target sends a disconnect association. After this
the target starts tearing down all resources and doesn't expect any
response.

Signed-off-by: Daniel Wagner <wagi@kernel.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>