nvmet-rdma: remove NVMET_RDMA_REQ_INVALIDATE_RKEY flag

We can simply use invalidate_rkey to check instead of adding a flag.

Signed-off-by: Guixin Liu <kanie@linux.alibaba.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Keith Busch <kbusch@kernel.org>