tracing: mark user_events as BROKEN

After being merged, user_events become more visible to a wider audience
that have concerns with the current API.

It is too late to fix this for this release, but instead of a full
revert, just mark it as BROKEN (which prevents it from being selected in
make config). Then we can work finding a better API. If that fails,
then it will need to be completely reverted.

To not have the code silently bitrot, still allow building it with
COMPILE_TEST.

And to prevent the uapi header from being installed, then later changed,
and then have an old distro user space see the old version, move the
header file out of the uapi directory.

Surround the include with CONFIG_COMPILE_TEST to the current location,
but when the BROKEN tag is taken off, it will use the uapi directory,
and fail to compile. This is a good way to remind us to move the header
back.

Link: https://lore.kernel.org/all/[e-mail]
Link: https://lkml.kernel.org/r/[e-mail]
Suggested-by: Mathieu Desnoyers <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/trace/Kconfig

···
 	bool "User trace events"
 	select TRACING
 	select DYNAMIC_EVENTS
+	depends on BROKEN || COMPILE_TEST # API needs to be straighten out
 	help
 	  User trace events are user-defined trace events that
 	  can be used like an existing kernel trace event.  User trace

kernel/trace/trace_events_user.c

···
 #include <linux/tracefs.h>
 #include <linux/types.h>
 #include <linux/uaccess.h>
+/* Reminder to move to uapi when everything works */
+#ifdef CONFIG_COMPILE_TEST
+#include <linux/user_events.h>
+#else
 #include <uapi/linux/user_events.h>
+#endif
 #include "trace.h"
 #include "trace_dynevent.h"