vxlan: move IGMP join/leave to work queue

Do join/leave from work queue to avoid lock inversion problems
between normal socket and RTNL. The code comes out cleaner
as well.

Uses Cong Wang's suggestion to turn refcnt into a real atomic
since now need to handle case where last use of socket is IGMP
worker.

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