vxlan: fix race caused by dropping rtnl_unlock

It is possible for two cpu's to race creating vxlan device.
For most cases this is harmless, but the ability to assign "next
avaliable vxlan device" relies on rtnl lock being held across the
whole operation. Therfore two instances of calling:
  ip li add vxlan%d vxlan ...
could collide and create two devices with same name.

To fix this defer creation of socket to a work queue, and
handle possible races there. Introduce a lock to ensure that
changes to vxlan socket hash list is SMP safe.

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