[kernel] Make grabbing an IST stack atomic

In the beginning of the interrupt handler, we had previously checked if
the current handler had grabbed an IST stack from the IDT/TSS. If it
was, it saved this value and set it to 0 in the IDT, then restored it at
the end.

Now this is an atomic action. This is unlikely to make a difference
unless the interrupt handler is itself interrupted by an exception
before being able to swap the IDT value, but such a situation is now
impossible.

src/kernel/interrupts.cpp

@@ -75,8 +75,6 @@ isr_handler(cpu_state *regs)
     // this stack
     IDT &idt = IDT::current();
     uint8_t old_ist = idt.get_ist(vector);
-    if (old_ist)
-        idt.set_ist(vector, 0);
 
     char message[200];
 
@@ -165,7 +163,7 @@ isr_handler(cpu_state *regs)
 
     // Return the IST for this vector to what it was
     if (old_ist)
-        idt.set_ist(vector, old_ist);
+        idt.return_ist(vector, old_ist);
     *reinterpret_cast<uint32_t *>(apic_eoi_addr) = 0;
 }