x86/fpu: Rename init_thread_xstate() to fpstate_xstate_init_size()

So init_thread_xstate() is a misnomer in that it's not really related to a specific
thread - it determines, once during initial bootup, the size of the xstate context.

Also improve the comments.
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

@@ -158,7 +158,7 @@ static void mxcsr_feature_mask_init(void)
 	mxcsr_feature_mask &= mask;
 }
 
-static void init_thread_xstate(void)
+static void fpstate_xstate_init_size(void)
 {
 	/*
 	 * Note that xstate_size might be overwriten later during
@@ -216,11 +216,11 @@ void fpu__cpu_init(void)
 	write_cr0(cr0);
 
 	/*
-	 * init_thread_xstate is only called once to avoid overriding
-	 * xstate_size during boot time or during CPU hotplug.
+	 * fpstate_xstate_init_size() is only called once, to avoid overriding
+	 * 'xstate_size' during (secondary CPU) bootup or during CPU hotplug.
 	 */
 	if (xstate_size == 0)
-		init_thread_xstate();
+		fpstate_xstate_init_size();
 
 	mxcsr_feature_mask_init();
 	xsave_init();