ceph: d_obtain_{alias,root}(ERR_PTR(...)) will do the right thing

Clean up the code.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>