net/hyperv: fix possible memory leak in do_set_multicast()

do_set_multicast() may not free the memory malloc in
netvsc_set_multicast_list().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>