rust: time: fix as_micros_ceil() to round correctly for negative Delta

The ceiling-division idiom `(n + d - 1) / d` only produces the
correct result when `n` is non-negative.

For example, if n = -1000 (exactly -1us), the old code computed (-1000
+ 999) / 1000 == 0 instead of -1.

For negative n, truncating division already rounds towards positive
infinity, so no bias is needed in that case.

Fixes: fae0cdc12340 ("rust: time: Introduce Delta type")
Signed-off-by: FUJITA Tomonori <fujita.tomonori@gmail.com>
Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260713225235.3243480-1-tomo@flapping.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

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