x86/fpu: Add comments to fpu__save() and restrict its export

Add an explanation to fpu__save() and also don't export it to
random modules - we don't want them to futz around with deep kernel
internals.

Reviewed-by: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/i387.c

@@ -117,6 +117,9 @@ void __kernel_fpu_end(void)
 }
 EXPORT_SYMBOL(__kernel_fpu_end);
 
+/*
+ * Save the FPU state (initialize it if necessary):
+ */
 void fpu__save(struct task_struct *tsk)
 {
 	preempt_disable();
@@ -130,7 +133,7 @@ void fpu__save(struct task_struct *tsk)
 	}
 	preempt_enable();
 }
-EXPORT_SYMBOL(fpu__save);
+EXPORT_SYMBOL_GPL(fpu__save);
 
 unsigned int mxcsr_feature_mask __read_mostly = 0xffffffffu;
 unsigned int xstate_size;