migration/rdma: clang compilation fix

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>