nvmet-rdma: replace ida_simple[get|remove] with the simler ida_[alloc|free]

ida_simple_[get|remove] are wrappers anyways.

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>