x86/fpu: Remove the free_thread_xstate() complication

Use fpstate_free() directly to manage FPU state.

Only process.c was using this method, so this is a speedup as well,
as it removes the extra function call and related clobbers.

Reviewed-by: Borislav Petkov <[e-mail]>
Cc: Andy Lutomirski <[e-mail]>
Cc: Dave Hansen <[e-mail]>
Cc: Fenghua Yu <[e-mail]>
Cc: H. Peter Anvin <[e-mail]>
Cc: Linus Torvalds <[e-mail]>
Cc: Oleg Nesterov <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Thomas Gleixner <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

arch/x86/include/asm/processor.h

···
 #endif	/* X86_64 */
 
 extern unsigned int xstate_size;
-extern void free_thread_xstate(struct task_struct *);
 extern struct kmem_cache *task_xstate_cachep;
 
 struct perf_event;

arch/x86/kernel/process.c

···
 	return 0;
 }
 
-void free_thread_xstate(struct task_struct *tsk)
-{
-	fpstate_free(&tsk->thread.fpu);
-}
-
 void arch_release_task_struct(struct task_struct *tsk)
 {
-	free_thread_xstate(tsk);
+	fpstate_free(&tsk->thread.fpu);
 }
 
 void arch_task_cache_init(void)
···
 	if (!use_eager_fpu()) {
 		/* FPU state will be reallocated lazily at the first use. */
 		drop_fpu(tsk);
-		free_thread_xstate(tsk);
+		fpstate_free(&tsk->thread.fpu);
 	} else {
 		if (!tsk_used_math(tsk)) {
 			/* kthread execs. TODO: cleanup this horror. */