Rollup merge of #102760 - saethlin:dont-reinit-buffer, r=Mark-Simulacrum

Avoid repeated re-initialization of the BufReader buffer

Fixes https://github.com/rust-lang/rust/issues/102727

We accidentally removed this in https://github.com/rust-lang/rust/pull/98748. It looks so redundant. But it isn't.

The default `Read::read_buf` will defensively initialize the whole buffer, if any of it is indicated to be uninitialized. In uses where reads from the wrapped `Read` impl completely fill the `BufReader`, `initialized` and `filled` are the same, and this extra member isn't required. But in the reported issue, the `BufReader` wraps a `Read` impl which will _never_ fill the whole buffer. So the default `Read::read_buf` implementation repeatedly re-initializes the extra space in the buffer.

This adds back the extra `initialized` member, which ensures that the default `Read::read_buf` only zero-initialized the buffer once, and I've tried to add a comment which explains this whole situation.

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