x86, mm: fault.c, use printk_once() in is_errata93()

Andrew pointed out that the 'once' variable has a needlessly
function-global scope. We can in fact eliminate it completely,
via the use of printk_once().

[ Impact: cleanup ]

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>