x86: add user_atomic_cmpxchg_inatomic at uaccess.h

This patch adds user_atomic_cmpxchg_inatomic() to use CMPXCHG
instruction against a user space address.

This generalizes the already existing futex_atomic_cmpxchg_inatomic()
so it can be used in other contexts.  This will be used in the
upcoming support for Intel MPX (Memory Protection Extensions.)

[ hpa: replaced #ifdef inside a macro with IS_ENABLED() ]

Signed-off-by: Qiaowei Ren <qiaowei.ren@intel.com>
Link: http://lkml.kernel.org/r/1387002303-6620-1-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>