net/ipv4/ip_vti.c: Fix __rcu warnings detected by sparse.

With CONFIG_SPARSE_RCU_POINTER=y sparse identified references which did not
specificy __rcu in ip_vti.c

Signed-off-by: Saurabh Mohan <saurabh.mohan@vyatta.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>