RDMA/rxe: Fix extra copy in prepare_ack_packet

Currently prepare_ack_packet writes almost all the fields of the BTH in
the ack packet twice. Replace code with the subroutine init_bth().

Fixes: 8700e3e ("Soft RoCE driver")
Link: https://lore.kernel.org/r/20210618045742.204195-6-rpearsonhpe@gmail.com
Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

drivers/infiniband/sw/rxe/rxe_resp.c

@@ -637,18 +637,11 @@ static struct sk_buff *prepare_ack_packet(struct rxe_qp *qp,
 	ack->opcode = opcode;
 	ack->mask = rxe_opcode[opcode].mask;
 	ack->paylen = paylen;
-
-	/* fill in bth using the request packet headers */
-	memcpy(ack->hdr, pkt->hdr, RXE_BTH_BYTES);
-
-	bth_set_opcode(ack, opcode);
-	bth_set_qpn(ack, qp->attr.dest_qp_num);
-	bth_set_pad(ack, pad);
-	bth_set_se(ack, 0);
-	bth_set_psn(ack, psn);
-	bth_set_ack(ack, 0);
 	ack->psn = psn;
 
+	bth_init(ack, opcode, 0, 0, pad, IB_DEFAULT_PKEY_FULL,
+		 qp->attr.dest_qp_num, 0, psn);
+
 	if (ack->mask & RXE_AETH_MASK) {
 		aeth_set_syn(ack, syndrome);
 		aeth_set_msn(ack, qp->resp.msn);