apparmor: fix an error code in __aa_create_ns()

We should return error pointers in this function.  Returning NULL
results in a NULL dereference in the caller.

Fixes: 73688d1ed0b8 ("apparmor: refactor prepare_ns() and make usable from different views")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: John Johansen <john.johansen@canonical.com>

security/apparmor/policy_ns.c

@@ -255,7 +255,7 @@ static struct aa_ns *__aa_create_ns(struct aa_ns *parent, const char *name,
 
 	ns = alloc_ns(parent->base.hname, name);
 	if (!ns)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 	ns->level = parent->level + 1;
 	mutex_lock_nested(&ns->lock, ns->level);
 	error = __aafs_ns_mkdir(ns, ns_subns_dir(parent), name, dir);