vmbus: re-enable channel tasklet

This problem shows up in 4.11 when netvsc driver is removed and reloaded.
The problem is that the channel is closed during module removal and the
tasklet for processing responses is disabled. When module is reloaded
the channel is reopened but the tasklet is marked as disabled.
The fix is to re-enable tasklet at the end of close which gets it back
to the initial state.

The issue is less urgent in 4.12 since network driver now uses NAPI
and not the tasklet; and other VMBUS devices are rarely unloaded/reloaded.

Fixes: dad72a1d ("vmbus: remove hv_event_tasklet_disable/enable")

Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>