ceph: avoid creating orphan object when checking pool permission

ceph: avoid creating orphan object when checking pool permission

Pool permission check needs to write to the first object. But for
snapshot, head of the first object may have already been deleted.
Skip the check for snapshot inode to avoid creating orphan object.

Link: http://tracker.ceph.com/issues/18211
Signed-off-by: Yan, Zheng <zyan@redhat.com>