apparmor: fix absroot causing audited secids to begin with =

apparmor: fix absroot causing audited secids to begin with =

AppArmor is prefixing secids that are converted to secctx with the =
to indicate the secctx should only be parsed from an absolute root
POV. This allows catching errors where secctx are reparsed back into
internal labels.

Unfortunately because audit is using secid to secctx conversion this
means that subject and object labels can result in a very unfortunate
== that can break audit parsing.

eg. the subj==unconfined term in the below audit message

type=USER_LOGIN msg=audit(1639443365.233:160): pid=1633 uid=0 auid=1000
ses=3 subj==unconfined msg='op=login id=1000 exe="/usr/sbin/sshd"
hostname=192.168.122.1 addr=192.168.122.1 terminal=/dev/pts/1 res=success'

Fix this by switch the prepending of = to a _. This still works as a
special character to flag this case without breaking audit. Also move
this check behind debug as it should not be needed during normal
operqation.

Fixes: d2e43ca03cb5 ("apparmor: add support for absolute root view based labels")
Reported-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>