x86/fpu: Introduce the x86_task_fpu() helper method

The per-task FPU context/save area is allocated right
next to task_struct, currently in a variable-size
array via task_struct::thread.fpu[], but we plan to
fully hide it from the C type scope.

Introduce the x86_task_fpu() accessor that gets to the
FPU context pointer explicitly from the task pointer.

Right now this is a simple (task)->thread.fpu wrapper.

Signed-off-by: Ingo Molnar <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Brian Gerst <[e-mail]>
Cc: Chang S. Bae <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/processor.h

···
 	 */
 };
 
+#define x86_task_fpu(task) (&(task)->thread.fpu)
+
 extern void fpu_thread_struct_whitelist(unsigned long *offset, unsigned long *size);
 
 static inline void arch_thread_struct_whitelist(unsigned long *offset,