ceph: cleanup splice_dentry()

splice_dentry() may drop the original dentry and return other
dentry. It relies on its caller to update pointer that points
to the dropped dentry. This is error-prone.

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