Revert "ipv6: Factorise ip6_route_multipath_add()."

Revert "ipv6: Factorise ip6_route_multipath_add()."

Commit 71c0efb6d12f ("ipv6: Factorise ip6_route_multipath_add().") split
a loop in ip6_route_multipath_add() so that we can put rcu_read_lock()
between ip6_route_info_create() and ip6_route_info_create_nh().

We no longer need to do so as ip6_route_info_create_nh() does not require
RCU now.

Let's revert the commit to simplify the code.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250516022759.44392-6-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>