x86: replace futex_atomic_cmpxchg_inatomic() with user_atomic_cmpxchg_inatomic

futex_atomic_cmpxchg_inatomic() is simply the 32-bit implementation of
user_atomic_cmpxchg_inatomic(), which in turn is simply a
generalization of the original code in
futex_atomic_cmpxchg_inatomic().

Use the newly generalized user_atomic_cmpxchg_inatomic() as the futex
implementation, too.

[ hpa: retain the inline in futex.h rather than changing it to a macro ]

Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
Link: http://lkml.kernel.org/r/1387002303-6620-2-git-send-email-qiaowei.ren@intel.com
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>