rpcrdma: Always release the rpcrdma_device's xa_array

In the Linux kernel, the following vulnerability has been resolved: rpcrdma: Always release the rpcrdma_device's xa_array Dai pointed out that the xa_init_flags() in rpcrdma_add_one() needs to have a matching xa_destroy() in rpcrdma_remove_one() to release underlying memory that the xarray might have accrued during operation.