kerncore: add must_use and inline(always)

The missing must_use was just an oversight -- access check functions
that return flags, instead of faulting or returning Result, should
always be must_use.

inline(always) turns out to significantly improve performance of
Task::can_access by specializing the generated code for the predicate it
uses.

sys/kerncore/src/lib.rs

@@ -152,6 +152,8 @@ impl<T: MemoryRegion> MemoryRegion for &T {
 /// that meet the `region_ok` condition.
 ///
 /// `false` otherwise.
+#[must_use]
+#[inline(always)]
 pub fn can_access<S, R>(
     slice: S,
     table: &[R],