x86: add context tag to mark mm when running a task in 32-bit compatibility mode

This tag is intended to mirror the thread info TIF_IA32 flag.  Will be used to
identify mm's which support 32 bit tasks running in compatibility mode without
requiring a reference to the task itself.

Signed-off-by: Stephen Wilson <wilsons@start.ca>
Reviewed-by: Michel Lespinasse <walken@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/x86/include/asm/mmu.h

@@ -13,6 +13,12 @@ typedef struct {
 	int size;
 	struct mutex lock;
 	void *vdso;
+
+#ifdef CONFIG_X86_64
+	/* True if mm supports a task running in 32 bit compatibility mode. */
+	unsigned short ia32_compat;
+#endif
+
 } mm_context_t;
 
 #ifdef CONFIG_SMP