Drivers: hv: don't leak memory in vmbus_establish_gpadl()

commit 7cc80c98 upstream.

In some cases create_gpadl_header() allocates submessages but we never
free them.

[sumits] Note for stable:
Upstream commit 4d637632:
(Drivers: hv: get rid of redundant messagecount in create_gpadl_header())
changes the list usage to initialize list header in all cases; that patch
isn't added to stable, so the current patch is modified a little bit from
the upstream commit to check if the list is valid or not.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>