hv_netvsc: Fix the receive buffer size limit

[ Upstream commit 11b2b653102571ac791885324371d9a1a17b900e ]

The max should be 31 MB on host with NVSP version > 2.

On legacy hosts (NVSP version <=2) only 15 MB receive buffer is allowed,
otherwise the buffer request will be rejected by the host, resulting
vNIC not coming up.

The NVSP version is only available after negotiation. So, we add the
limit checking for legacy hosts in netvsc_init_buf().

Fixes: 5023a6db ("netvsc: increase default receive buffer size")
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>