kernel/thread: Fix behavior of {get|set}_thread_affinity.

* Handle threads without masks: this means they can run on any CPU.

 * Return only enabled CPUs from get_thread_affinity.

 * Clear the user mask's extra bits.

 * Allow masks with more CPUs set than are active, same as Linux,
   but disallow masks with no intersection with the active CPUs.

 * If setting to all enabled CPUs, clear the mask, so that if
   more CPUs are enabled in the future, this can run on them too.

 * Add permissions checks.

Fixes #20251.
(cherry picked from commit f46b3483057b149179dda7abdcc2dde38994a8cc)

Change-Id: Ib8649cb0a252fc8d9cfca691c2e03748561db741
Reviewed-on: https://review.haiku-os.org/c/haiku/+/11529
Reviewed-by: waddlesplash <[email protected]>

headers/private/kernel/smp.h

@@ -58,10 +58,12 @@ public:
 
 	inline	bool		Matches(const CPUSet& mask) const;
 	inline	CPUSet		And(const CPUSet& mask) const;
+	inline	bool		operator==(const CPUSet& other) const;
 
 	inline	bool		IsEmpty() const;
 
 	inline uint32		Bits(uint32 index) const { return fBitmap[index];}
+
 private:
 	static	const int	kArrayBits = 32;
 	static	const int	kArraySize = ROUNDUP(SMP_MAX_CPUS, kArrayBits) / kArrayBits;
@@ -199,6 +201,18 @@ CPUSet::Matches(const CPUSet& mask) const
 }
 
 
+inline bool
+CPUSet::operator==(const CPUSet& other) const
+{
+	for (int i = 0; i < kArraySize; i++) {
+		if (fBitmap[i] != other.fBitmap[i])
+			return false;
+	}
+
+	return true;
+}
+
+
 inline bool
 CPUSet::IsEmpty() const
 {