vxlan: reduce usage of synchronize_net in ndo_stop

vxlan: reduce usage of synchronize_net in ndo_stop

We only need to do the synchronize_net dance once for both, ipv4 and
ipv6 sockets, thus removing one synchronize_net in case both sockets get
dismantled.

Orabug: 29755932

Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 544a773a01828e3cc3b553721f68d880d0d27a97)

Reviewed-by: Rama Nichanamatlu <rama.nichanamatlu@oracle.com>
Signed-off-by: Venkat Venkatsubra <venkat.x.venkatsubra@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>