nvme-rdma: fix double freeing of async event data

Some error paths in configuration of admin queue free data buffer
associated with async request SQE without resetting the data buffer
pointer to NULL, This buffer is also freed up again if the controller
is shutdown or reset.

Signed-off-by: Prabhath Sajeepa <psajeepa@purestorage.com>
Reviewed-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>