x86/entry/32: Convert IRET exception to IDTENTRY_SW

Convert the IRET exception handler to IDTENTRY_SW. This is slightly
different than the conversions of hardware exceptions as the IRET exception
is invoked via an exception table when IRET faults. So it just uses the
IDTENTRY_SW mechanism for consistency. It does not emit ASM code as it does
not fit the other idtentry exceptions.

  - Implement the C entry point with DEFINE_IDTENTRY_SW() which maps to
    DEFINE_IDTENTRY()
  - Fixup the XEN/PV code
  - Remove the old prototypes
  - Remove the RCU warning as the new entry macro ensures correctness

No functional change.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505134906.128769226@linutronix.de

arch/x86/xen/xen-asm_32.S

@@ -117,7 +117,7 @@ iret_restore_end:
 
 1:	iret
 xen_iret_end_crit:
-	_ASM_EXTABLE(1b, iret_exc)
+	_ASM_EXTABLE(1b, asm_iret_error)
 
 hyper_iret:
 	/* put this out of line since its very rarely used */