Drivers: hv: vmbus: Copy the hv_message in vmbus_on_msg_dpc()

Drivers: hv: vmbus: Copy the hv_message in vmbus_on_msg_dpc()

Since the message is in memory shared with the host, an erroneous or a
malicious Hyper-V could 'corrupt' the message while vmbus_on_msg_dpc()
or individual message handlers are executing.  To prevent it, copy the
message into private memory.

Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20201209070827.29335-4-parri.andrea@gmail.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>