x86: atomic64: Clean up atomic64_sub_and_test() and atomic64_add_negative()

Linus noticed that the variable name 'old_val' is
confusingly named in these functions - the correct
naming is 'new_val'.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
LKML-Reference: <alpine.LFD.2.01.0907030942260.3210@localhost.localdomain>
Signed-off-by: Ingo Molnar <mingo@elte.hu>