Remove casting the values returned by memory allocation function.

Coccinelle emits WARNING: casting value returned by memory allocation
unction to (struct roce_destroy_qp_req_output_params *) is useless.

This issue was detected by using the Coccinelle software.
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>