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: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NJohn Johansen <john.johansen@canonical.com>