We accidentally return ERR_PTR(0) which is NULL.  The caller isn't
explicitly checking for that but I couldn't immediately spot whether
this would lead to a NULL dereference.  Anyway, we can fix add an
error code easily enough.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NChristoph Hellwig <hch@lst.de>