x86/asm: Clobber flags in clear_page()

All clear_page() implementations use XOR which resets flags.

Judging by allyesconfig disassembly no code is affected.
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20180113185048.GA23111@avx2Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/page_64.h

@@ -47,7 +47,7 @@ static inline void clear_page(void *page)
 			   clear_page_erms, X86_FEATURE_ERMS,
 			   "=D" (page),
 			   "0" (page)
-			   : "memory", "rax", "rcx");
+			   : "cc", "memory", "rax", "rcx");
 }
 
 void copy_page(void *to, void *from);