hv_utils: drain the timesync packets on onchannelcallback

commit b46b4a8a

 upstream.

There could be instances where a system stall prevents the timesync
packets to be consumed. And this might lead to more than one packet
pending in the ring buffer. Current code empties one packet per callback
and it might be a stale one. So drain all the packets from ring buffer
on each callback.

Signed-off-by: Vineeth Pillai <viremana@linux.microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20200821152849.99517-1-viremana@linux.microsoft.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>
[ The old code in the upstream commit uses HV_HYP_PAGE_SIZE, but
  the old code in 5.4.y sitll uses PAGE_SIZE. Fixed this manually for 5.4.y.
  Note: 5.4.y already has the define HV_HYP_PAGE_SIZE, so the new code in
  in the upstream commit works for 5.4.y.
  If there are multiple messages in the host-to-guest ringbuffer of the TimeSync
  device, 5.4.y only handles 1 message, and later the host puts new messages
  into the ringbuffer without signaling the guest because the ringbuffer is not
  empty, causing a "hung" ringbuffer. Backported the mainline fix for this issue.]
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>