bpf: iter_udp: Retry with a larger batch size without going back to the previous bucket

The current logic is to use a default size 16 to batch the whole bucket.
If it is too small, it will retry with a larger batch size.

The current code accidentally does a state->bucket-- before retrying.
This goes back to retry with the previous bucket which has already
been done. This patch fixed it.

It is hard to create a selftest. I added a WARN_ON(state->bucket < 0),
forced a particular port to be hashed to the first bucket,
created >16 sockets, and observed the for-loop went back
to the "-1" bucket.

Cc: Aditi Ghag <aditi.ghag@isovalent.com>
Fixes: c96dac8d36 ("bpf: udp: Implement batching for sockets iterator")
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Aditi Ghag <aditi.ghag@isovalent.com>
Link: https://lore.kernel.org/r/20240112190530.3751661-2-martin.lau@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

net/ipv4/udp.c

@@ -3213,7 +3213,6 @@ static struct sock *bpf_iter_udp_batch(struct seq_file *seq)
 		/* After allocating a larger batch, retry one more time to grab
 		 * the whole bucket.
 		 */
-		state->bucket--;
 		goto again;
 	}
 done: