x86: remove volatile keyword from clflush.

the p parameter is an explicit memory reference, and is
enough to prevent gcc to being nasty here. The volatile
seems completely not needed.

Signed-off-by: Glauber de Oliveira Costa <gcosta@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>