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: fae0cdc123 ("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>

rust/kernel/time.rs

@@ -441,15 +441,22 @@ pub const fn as_nanos(self) -> i64 {
     /// to the value in the [`Delta`].
     #[inline]
     pub fn as_micros_ceil(self) -> i64 {
+        let n = self.as_nanos();
+        let n = if n >= 0 {
+            n.saturating_add(NSEC_PER_USEC - 1)
+        } else {
+            n
+        };
+
         #[cfg(CONFIG_64BIT)]
         {
-            self.as_nanos().saturating_add(NSEC_PER_USEC - 1) / NSEC_PER_USEC
+            n / NSEC_PER_USEC
         }
 
         #[cfg(not(CONFIG_64BIT))]
         // SAFETY: It is always safe to call `ktime_to_us()` with any value.
         unsafe {
-            bindings::ktime_to_us(self.as_nanos().saturating_add(NSEC_PER_USEC - 1))
+            bindings::ktime_to_us(n)
         }
     }