ipv4: fib: Make fib_info_hashfn() return struct hlist_head.

ipv4: fib: Make fib_info_hashfn() return struct hlist_head.

Every time fib_info_hashfn() returns a hash value, we fetch
&fib_info_hash[hash].

Let's return the hlist_head pointer from fib_info_hashfn() and
rename it to fib_info_hash_bucket() to match a similar function,
fib_info_laddrhash_bucket().

Note that we need to move the fib_info_hash assignment earlier in
fib_info_hash_move() to use fib_info_hash_bucket() in the for loop.

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/20250228042328.96624-5-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>