net/ipv6: Fix gfp_flags arg to addrconf_prefix_route

Eric noticed that __ipv6_ifa_notify is called under rcu_read_lock, so
the gfp argument to addrconf_prefix_route can not be GFP_KERNEL.

While scrubbing other calls I noticed addrconf_addr_gen has one
place with GFP_ATOMIC that can be GFP_KERNEL.

Fixes: acb54e3c ("net/ipv6: Add gfp_flags to route add functions")
Reported-by:  <syzbot+2add39b05179b31f912f@syzkaller.appspotmail.com>
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>