ipv4: fib: Allocate fib_info_hash[] during netns initialisation.

ipv4: fib: Allocate fib_info_hash[] during netns initialisation.

We will allocate fib_info_hash[] and fib_info_laddrhash[] for each netns.

Currently, fib_info_hash[] is allocated when the first route is added.

Let's move the first allocation to a new __net_init function.

Note that we must call fib4_semantics_exit() in fib_net_exit_batch()
because ->exit() is called earlier than ->exit_batch().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20250228042328.96624-4-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>