ipv6: get rid of ipip6_prl_lock

As noticed by Julia Lawall, ipip6_tunnel_add_prl() incorrectly calls 
kzallloc(..., GFP_KERNEL) while a spinlock is held. She provided
a patch to use GFP_ATOMIC instead.

One possibility would be to convert this spinlock to a mutex, or
preallocate the thing before taking the lock.

After RCU conversion, it appears we dont need this lock, since 
caller already holds RTNL

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>