ipv6: Pass gfp_flags down to ip6_route_info_create_nh().

Since commit c4837b98 ("ipv6: Split ip6_route_info_create()."),
ip6_route_info_create_nh() uses GFP_ATOMIC as it was expected to be
called under RCU.

Now, we can call it without RCU and use GFP_KERNEL.

Let's pass gfp_flags to ip6_route_info_create_nh().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250516022759.44392-7-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>