ceph: fix warning PTR_ERR_OR_ZERO can be used

change1: fix below warning  reported by coccicheck

/fs/ceph/export.c:371:33-39: WARNING: PTR_ERR_OR_ZERO can be used

change2: typecasted PTR_ERR_OR_ZERO to long as dout expecting long

Signed-off-by: Hariprasad Kelam <hariprasad.kelam@gmail.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>