garp: remove one synchronize_rcu() call

Speedup vlan dismantling in CONFIG_VLAN_8021Q_GVRP=y cases,
by using a call_rcu() to free the memory instead of waiting with
expensive synchronize_rcu() [ while RTNL is held ]

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ben Greear <greearb@candelatech.com>
Cc: Patrick McHardy <kaber@trash.net>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>