[V1] Fix when max_model_len is not divisible by block_size

[WoosukKwon]

Fixes #10891

Thanks @ywang96 for investigating the bug and pinpointing the source of the bug.

[github-actions]

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[ywang96]

I've tested this PR for both text models and image models and the change looks good to me!

As discussed offline, we should add some unit tests for V1 engine around issue like this, since the bug only occur in certain situations without an explicit error.