migration/rdma: zore out head.repeat to make the error more clear

Previously, we got a confusion error that complains
the RDMAControlHeader.repeat:
qemu-system-x86_64: rdma: Too many requests in this message (3638950032).Bailing.

Actually, it's caused by an unexpected RDMAControlHeader.type.
After this patch, error will become:
qemu-system-x86_64: Unknown control message QEMU FILE

Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <20230926100103.201564-2-lizhijian@fujitsu.com>

migration/rdma.c

@@ -2831,7 +2831,7 @@ static ssize_t qio_channel_rdma_writev(QIOChannel *ioc,
         size_t remaining = iov[i].iov_len;
         uint8_t * data = (void *)iov[i].iov_base;
         while (remaining) {
-            RDMAControlHeader head;
+            RDMAControlHeader head = {};
 
             len = MIN(remaining, RDMA_SEND_INCREMENT);
             remaining -= len;