x86/gdt: Use bitfields for initialization

The GDT entry related code uses two ways to access entries via
union fields:

 - bitfields

 - macros which initialize the two 16-bit parts of the entry
   by magic shift and mask operations.

Clean it up and only use the bitfields to initialize and access entries.

( The old access patterns were partly done due to GCC optimizing bitfield
  accesses in a horrible way - that's mostly fixed these days and clarity
  of code in such low level accessors is very important. )

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20170828064958.197673367@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/vdso/vma.c

@@ -351,7 +351,7 @@ static void vgetcpu_cpu_init(void *arg)
 	 * and 8 bits for the node)
 	 */
 	d.limit0 = cpu | ((node & 0xf) << 12);
-	d.limit = node >> 4;
+	d.limit1 = node >> 4;
 	d.type = 5;		/* RO data, expand down, accessed */
 	d.dpl = 3;		/* Visible to user code */
 	d.s = 1;		/* Not a system segment */