ipv6: Preallocate nhc_pcpu_rth_output in ip6_route_info_create().

ipv6: Preallocate nhc_pcpu_rth_output in ip6_route_info_create().

ip6_route_info_create_nh() will be called under RCU.

It calls fib_nh_common_init() and allocates nhc->nhc_pcpu_rth_output.

As with the reason for rt->fib6_nh->rt6i_pcpu, we want to avoid
GFP_ATOMIC allocation for nhc->nhc_pcpu_rth_output under RCU.

Let's preallocate it in ip6_route_info_create().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250418000443.43734-9-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>