gtp: remove useless rcu_read_lock()

The rtnl lock is taken just the line above, no need to take the rcu also.

Fixes: 1788b856

 ("gtp: fix use-after-free in gtp_encap_destroy()")
Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>