hv_netvsc: Mark VF as slave before exposing it to user-mode

When a VF is being exposed form the kernel, it should be marked as "slave"
before exposing to the user-mode. The VF is not usable without netvsc
running as master. The user-mode should never see a VF without the "slave"
flag.

This commit moves the code of setting the slave flag to the time before
VF is exposed to user-mode.

Cc: stable@vger.kernel.org
Fixes: 0c195567

 ("netvsc: transparent VF management")
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>