When "multipath -f" is run on a device that doesn't exist or isn't a
multipath device, the command will not remove any device, but it will
still return success. Multiple functions rely on _dm_flush_map()
returning success when called with name that doesn't match any
multipath device. So before calling _dm_flush_map(), call dm_is_mpath(),
to check if the device exists and is a multipath device, and return
failure if it's not.
