RDMA/mlx5: Initialize return variable in case pagefault was skipped

Pagefaults occurred in non-ODP MR are completely valid events, so
initialize return variable to 0.

Fixes: 4d5422a3 ("IB/mlx5: Skip non-ODP MR when handling a page fault")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>