file_contexts

Use the default filesystem label from genfs_contexts for the directory
/sys/kernel/debug/tracing and /sys/kernel/tracing, instead of explicitly
attempting to relabel it.

There are three cases we need to consider:

1) Old-style tracing functionality is on debugfs
2) tracing functionality is on tracefs, but mounted under debugfs
3) tracefs is mounted at /sys/kernel/tracing

For #1, the label on /sys/kernel/debug/tracing will be debugfs, and all
processes are allowed debugfs:dir search, so having the label be debugfs
instead of debugfs_tracing will not result in any permission change.

For #2, the label on /sys/kernel/debug/tracing will be debugfs_tracing,
which is the same as it is today. The empty directory
/sys/kernel/tracing wlll retain the sysfs label, avoiding the denial
below.

For #3, /sys/kernel/debug/tracing won't exist, and /sys/kernel/tracing
will have the debugfs_tracing label, where processes are allowed search
access.

Addresses the following denial:

avc:  denied  { associate } for  pid=1 comm="init" name="tracing"
dev="sysfs" ino=95 scontext=u:object_r:debugfs_tracing:s0
tcontext=u:object_r:sysfs:s0 tclass=filesystem permissive=0

Bug: 31856701
Bug: 35197529
Test: no denials on boot
Change-Id: I7233ea92c6987b8edfce9c2f1d77eb25c7df820f