APPARMOR: Fix memory leak of apparmor_init()

set_init_cxt() allocted sizeof(struct aa_task_cxt) bytes for cxt,
if register_security() failed, it will cause memory leak.

Signed-off-by: Zhitong Wang <zhitong.wangzt@alibaba-inc.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: James Morris <jmorris@namei.org>