selinux: add security in-core xattr support for tracefs

Since kernel 4.1 ftrace is supported as a new separate filesystem. It
gets automatically mounted by the kernel under the old path
/sys/kernel/debug/tracing. Because it lives now on a separate filesystem
SELinux needs to be updated to also support setting SELinux labels
on tracefs inodes.  This is required for compatibility in Android
when moving to Linux 4.1 or newer.

Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>
Signed-off-by: William Roberts <william.c.roberts@intel.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>