apparmor: audit_cap dedup based on subj_cred instead of profile

The previous audit_cap cache deduping was based on the profile that was
being audited. This could cause confusion due to the deduplication then
occurring across multiple processes, which could happen if multiple
instances of binaries matched the same profile attachment (and thus ran
under the same profile) or a profile was attached to a container and its
processes.

Instead, perform audit_cap deduping over ad->subj_cred, which ensures the
deduping only occurs across a single process, instead of across all
processes that match the current one's profile.

Signed-off-by: Ryan Lee <ryan.lee@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>