x86/idt: Cleanup trap_init()

No point in having all the IDT cruft in trap_init(). Move it into the IDT
code and fixup the comments.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200528145522.992376498@linutronix.de

arch/x86/kernel/traps.c

@@ -1055,15 +1055,6 @@ void __init trap_init(void)
 
 	idt_setup_traps();
 
-	/*
-	 * Set the IDT descriptor to a fixed read-only location, so that the
-	 * "sidt" instruction will not leak the location of the kernel, and
-	 * to defend the IDT against arbitrary memory write vulnerabilities.
-	 * It will be reloaded in cpu_init() */
-	cea_set_pte(CPU_ENTRY_AREA_RO_IDT_VADDR, __pa_symbol(idt_table),
-		    PAGE_KERNEL_RO);
-	idt_descr.address = CPU_ENTRY_AREA_RO_IDT;
-
 	/*
 	 * Should be a barrier for any external CPU state:
 	 */