misc/vmw_vmci: fix an infoleak in vmci_host_do_receive_datagram()

`struct vmci_event_qp` allocated by qp_notify_peer() contains padding,
which may carry uninitialized data to the userspace, as observed by
KMSAN:

  BUG: KMSAN: kernel-infoleak in instrument_copy_to_user ./include/linux/instrumented.h:121
   instrument_copy_to_user ./include/linux/instrumented.h:121
   _copy_to_user+0x5f/0xb0 lib/usercopy.c:33
   copy_to_user ./include/linux/uaccess.h:169
   vmci_host_do_receive_datagram drivers/misc/vmw_vmci/vmci_host.c:431
   vmci_host_unlocked_ioctl+0x33d/0x43d0 drivers/misc/vmw_vmci/vmci_host.c:925
   vfs_ioctl fs/ioctl.c:51
  ...

  Uninit was stored to memory at:
   kmemdup+0x74/0xb0 mm/util.c:131
   dg_dispatch_as_host drivers/misc/vmw_vmci/vmci_datagram.c:271
   vmci_datagram_dispatch+0x4f8/0xfc0 drivers/misc/vmw_vmci/vmci_datagram.c:339
   qp_notify_peer+0x19a/0x290 drivers/misc/vmw_vmci/vmci_queue_pair.c:1479
   qp_broker_attach drivers/misc/vmw_vmci/vmci_queue_pair.c:1662
   qp_broker_alloc+0x297...