x86: reintroduce volatile keyword in prototype to clflush()

The volatile keyword was removed from the clflush() prototype
in commit e34907ae; the comment there
states:

    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.

This reflects incorrect understanding of the function of the volatile
keyword there.  The purpose of the volatile keyword is informing gcc
that it is safe to pass a volatile pointer to this function.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>