um: Don't use generic barrier.h

UML has its own platform-specific barrier.h under arch/x86/um/,
which should get used. Fix the build system to use it, and then
fix the barrier.h to actually compile.
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

arch/um/include/asm/Kbuild

 # SPDX-License-Identifier: GPL-2.0
-generic-y += barrier.h
 generic-y += bpf_perf_event.h
 generic-y += bug.h
 generic-y += compat.h

arch/x86/um/asm/barrier.h

@@ -2,14 +2,7 @@
 #ifndef _ASM_UM_BARRIER_H_
 #define _ASM_UM_BARRIER_H_
 
-#include <asm/asm.h>
-#include <asm/segment.h>
-#include <asm/cpufeatures.h>
-#include <asm/cmpxchg.h>
-#include <asm/nops.h>
-
-#include <linux/kernel.h>
-#include <linux/irqflags.h>
+#include <asm/alternative.h>
 
 /*
  * Force strict CPU ordering.