[kernel] Don't double-construct the scheduler

The scheduler singleton was getting constructed twice, once at static
time and then again in main(). Make the singleton a pointer so we only
construct it once.

src/kernel/main.cpp

@@ -174,7 +174,7 @@ kernel_main(args::header *header)
 	devices.init_drivers();
 
 	syscall_enable();
-	scheduler *sched = new (&scheduler::get()) scheduler(devices.get_lapic());
+	scheduler *sched = new scheduler(devices.get_lapic());
 
 	sched->create_kernel_task(logger_task, scheduler::max_priority-1, true);
 

src/kernel/scheduler.cpp

@@ -16,7 +16,7 @@
 #include "elf/elf.h"
 #include "kutil/assert.h"
 
-scheduler scheduler::s_instance(nullptr);
+scheduler *scheduler::s_instance = nullptr;
 
 const uint64_t rflags_noint = 0x002;
 const uint64_t rflags_int = 0x202;
@@ -34,6 +34,9 @@ scheduler::scheduler(lapic *apic) :
 	m_clock(0),
 	m_last_promotion(0)
 {
+	kassert(!s_instance, "Multiple schedulers created!");
+	s_instance = this;
+
 	page_table *pml4 = page_manager::get_pml4();
 	process *kp = new process(pml4);
 	m_kernel_process = kp;

src/kernel/scheduler.h

@@ -73,7 +73,7 @@ public:
 
 	/// Get a reference to the system scheduler
 	/// \returns  A reference to the global system scheduler
-	static scheduler & get() { return s_instance; }
+	static scheduler & get() { return *s_instance; }
 
 private:
 	friend uintptr_t syscall_dispatch(uintptr_t, cpu_state &);
@@ -105,6 +105,6 @@ private:
 	uint64_t m_clock = 0;
 	uint64_t m_last_promotion;
 
-	static scheduler s_instance;
+	static scheduler *s_instance;
 };