RDMA/core: Convert to use ERR_CAST()

JIRA: https://issues.redhat.com/browse/RHEL-110100

commit 41e2649
Author: Li Haoran <li.haoran7@zte.com.cn>
Date:   Tue Apr 1 21:08:40 2025 +0800

    RDMA/core: Convert to use ERR_CAST()

    As opposed to open-code, using the ERR_CAST macro clearly indicates that
    this is a pointer to an error value and a type conversion was performed.

    Link: https://patch.msgid.link/r/20250401210840146_IyrV3zlejzz3eAnDmMSB@zte.com.cn
    Signed-off-by: Li Haoran <li.haoran7@zte.com.cn>
    Signed-off-by: Shao Mingyin <shao.mingyin@zte.com.cn>
    Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

Signed-off-by: Kamal Heib <kheib@redhat.com>

Author: Kamalheib

drivers/infiniband/core/mad_rmpp.c

@@ -158,7 +158,7 @@ static struct ib_mad_send_buf *alloc_response_msg(struct ib_mad_agent *agent,
 	ah = ib_create_ah_from_wc(agent->qp->pd, recv_wc->wc,
 				  recv_wc->recv_buf.grh, agent->port_num);
 	if (IS_ERR(ah))
-		return (void *) ah;
+		return ERR_CAST(ah);
 
 	hdr_len = ib_get_mad_data_offset(recv_wc->recv_buf.mad->mad_hdr.mgmt_class);
 	msg = ib_create_send_mad(agent, recv_wc->wc->src_qp,