net/netvsc: manage VF port under read/write lock

net/netvsc: manage VF port under read/write lock

With multiple channels, the primary channel may receive notification
that VF has been added or removed while secondary channel is in
process of doing receive or transmit.  Resolve this race by converting
existing vf_lock to a reader/writer lock.

Users of lock (tx/rx/stats) acquire for read, and actions like
add/remove acquire it for write.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>