hv_netvsc: split sub-channel setup into async and sync

When doing device hotplug the sub channel must be async to avoid
deadlock issues because device is discovered in softirq context.

When doing changes to MTU and number of channels, the setup
must be synchronous to avoid races such as when MTU and device
settings are done in a single ip command.

Reported-by: Thomas Walker <Thomas.Walker@twosigma.com>
Fixes: 8195b139 ("hv_netvsc: fix deadlock on hotplug")
Fixes: 732e4985

 ("netvsc: fix race on sub channel creation")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>