discover/devmapper: Retry dm-device remove if busy

discover/devmapper: Retry dm-device remove if busy

Buildroot's libdm is not built with --enable-udev_sync, so device-mapper
actions are not able to sync or wait for udev events.
(see 185676316, "discover/devmapper: Disable libdm udev sync support")

This can cause an issue when tearing down a snapshot in
devmapper_destroy_snapshot() which performs a DM_DEVICE_REMOVE task
against the snapshot, origin, and base devices one after the other. In
some cases if the interval between these actions is too short the action
can fail as the preceding device hasn't disappeared yet and the device
being removed is still busy.

Since we don't yet have a way to tell exactly when the device is ready,
pause for a short time and retry the action, letting
devmapper_destroy_snapshot() continue and, for example, letting
mount_device() fall back to the physical device.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>