migration/rdma: clang compilation fix

Configuring QEMU with:
        ../configure --cc=clang --enable-rdma

Leads to compilation error:

  CC      migration/rdma.o
  CC      migration/block.o
  qemu/migration/rdma.c:3615:58: error: taking address of packed member 'rkey' of class or structure
      'RDMARegisterResult' may result in an unaligned pointer value [-Werror,-Waddress-of-packed-member]
                            (uintptr_t)host_addr, NULL, &reg_result->rkey,
                                                         ^~~~~~~~~~~~~~~~
Fix it by using a temp local variable.

Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <20190304184923.24215-1-marcel.apfelbaum@gmail.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

migration/rdma.c

@@ -3613,13 +3613,16 @@ static int qemu_rdma_registration_handle(QEMUFile *f, void *opaque)
                 }
                 chunk_start = ram_chunk_start(block, chunk);
                 chunk_end = ram_chunk_end(block, chunk + reg->chunks);
+                /* avoid "-Waddress-of-packed-member" warning */
+                uint32_t tmp_rkey = 0;
                 if (qemu_rdma_register_and_get_keys(rdma, block,
-                            (uintptr_t)host_addr, NULL, &reg_result->rkey,
+                            (uintptr_t)host_addr, NULL, &tmp_rkey,
                             chunk, chunk_start, chunk_end)) {
                     error_report("cannot get rkey");
                     ret = -EINVAL;
                     goto out;
                 }
+                reg_result->rkey = tmp_rkey;
 
                 reg_result->host_addr = (uintptr_t)block->local_host_addr;