PopulateDeviceInfo() in pkg/clusterd/disk.go returns nil as *sys.LocalDisk
if an error has occurred. This causes a nil pointer exception at
getAvailableDevices() in pkg/daemon/ceph/osd/daemon.go. At least the returned
value should be checked at the caller.
I added an explicit error to the return value of PopulateDeviceInfo(). This
naturally revokes an error check at the caller.
I modified PopulateDeviceUdevInfo() in the same file too.
Signed-off-by: morimoto-cybozu <kenji_morimoto@cybozu.co.jp>