audit: Provide helper for dropping mark's chunk reference

Provide a helper function audit_mark_put_chunk() for dropping mark's
reference (which has to happen only after RCU grace period expires).
Currently that happens only from a single place but in later patches we
introduce more callers.

Reviewed-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Paul Moore <paul@paul-moore.com>