nvmet-rdma: drain the queue-pair just before freeing it

draining the qp right after disconnect might not suffice because
the nvmet sq is not fully drained (in nvmet_sq_destroy) and we might
see completions after the drain. Instead, drain right before the
qp destroy which comes after the sq destruction and we can be sure
that no posts come after the drain.

Tested-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>