vmbus: hvsock: add proper sync for vmbus_hvsock_device_unregister()

vmbus: hvsock: add proper sync for vmbus_hvsock_device_unregister()

Without the patch, vmbus_hvsock_device_unregister() can destroy the device
prematurely when close() is called, and can cause NULl dereferencing or
potential data loss (the last portion of the data stream may be dropped
prematurely).

Signed-off-by: Dexuan Cui <[email protected]>
Cc: Haiyang Zhang <[email protected]>
Cc: Stephen Hemminger <[email protected]>
Signed-off-by: K. Y. Srinivasan <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>