nvme: avoid double free special payload

If a discard request needs to be retried, and that retry may fail before
a new special payload is added, a double free will result. Clear the
RQF_SPECIAL_LOAD when the request is cleaned.

Signed-off-by: Chunguang Xu <chunguang.xu@shopee.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>