ipv4: fib: Fix fib_info_hash_alloc() allocation type

In preparation for making the kmalloc family of allocators type aware,
we need to make sure that the returned type from the allocation matches
the type of the variable being assigned. (Before, the allocator would
always return "void *", which can be implicitly cast to any pointer type.)

This was allocating many sizeof(struct hlist_head *) when it actually
wanted sizeof(struct hlist_head). Luckily these are the same size.
Adjust the allocation type to match the assignment.

Signed-off-by: Kees Cook <kees@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://patch.msgid.link/20250426060529.work.873-kees@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>