Add initial debugfs labeling support and label /sys/kernel/debug/tracing/trace_marker

Add initial support for labeling files on /sys/kernel/debug.
The kernel support was added in https://android-review.googlesource.com/122130
but the userspace portion of the change was never completed until now.

Start labeling the file /sys/kernel/debug/tracing/trace_marker . This
is the trace_marker file, which is written to by almost all processes
in Android. Allow global write access to this file.

This change should be submitted at the same time as the system/core
commit with the same Change-Id as this patch.

Change-Id: Id1d6a9ad6d0759d6de839458890e8cb24685db6d