um: Skip futex_atomic_cmpxchg_inatomic() test

futex_atomic_cmpxchg_inatomic() does not work on UML because
it triggers a copy_from_user() in kernel context.
On UML copy_from_user() can only be used if the kernel was called
by a real user space process such that UML can use ptrace()
to fetch the value.
Reported-by: NMiklos Szeredi <miklos@szeredi.hu>
Suggested-by: NGeert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: NRichard Weinberger <richard@nod.at>
Tested-by: NDaniel Walter <d.walter@0x90.at>

arch/um/Kconfig.common

@@ -3,6 +3,7 @@ config UML
 	default y
 	select HAVE_ARCH_AUDITSYSCALL
 	select HAVE_UID16
+	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select GENERIC_IRQ_SHOW
 	select GENERIC_CPU_DEVICES
 	select GENERIC_IO