apparmor: refactor/cleanup cred helper fns.

apparmor: refactor/cleanup cred helper fns.

aa_cred_raw_label() and cred_label() now do the same things so
consolidate to cred_label()

Document the crit section use and constraints better and refactor
__begin_current_label_crit_section() into a base fn
__begin_cred_crit_section() and a wrapper that calls the base with
current cred.

Reviewed-by: Georgia Garcia <georgia.garcia@canonical.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>