x86: align to clflush size

Do it instead of using the conservative approach we're currently
doing. This is the way x86_64 does, and this patch makes this piece
of code the same between them, ready to be integrated.

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