selinux: avtab_init() and cond_policydb_init() return void

The avtab_init() and cond_policydb_init() functions always return
zero so mark them as returning void and update the callers not to
check for a return value.

Suggested-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Reviewed-by: Ondrej Mosnacek <omosnace@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>