hyperv: Fix error return code in netvsc_init_buf()

Fix to return -ENOMEM from the kalloc error handling
case instead of 0.

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