nvme-fc: set NVME_REQ_CANCELLED in nvme_fc_terminate_exchange()

nvme_fc_terminate_exchange() is being called when exchanges are
being deleted, and as such we should be setting the NVME_REQ_CANCELLED
flag to have identical behaviour on all transports.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: James Smart <jsmart2021@gmail.com>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>