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: 73688d1e

 ("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>