vxlan: re-ignore EADDRINUSE from igmp_join

Before 56ef9c90[1] it used to ignore all errors from igmp_join().
That commit enhanced that and made it error out whatever error happened
with igmp_join(), but that's not good because when using multicast
groups vxlan will try to join it multiple times if the socket is reused
and then the 2nd and further attempts will fail with EADDRINUSE.

As we don't track to which groups the socket is already subscribed, it's
okay to just ignore that error.

Fixes: 56ef9c90 ("vxlan: Move socket initialization to within rtnl scope")
Reported-by: John Nielsen <lists@jnielsen.net>
Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>