dm init: ensure device probing has finished in dm-mod.waitfor=

dm init: ensure device probing has finished in dm-mod.waitfor=

The early_lookup_bdev() function returns successfully when the disk
device is present but not necessarily its partitions. In this situation,
dm_early_create() fails as the partition block device does not exist
yet.

In my case, this phenomenon occurs quite often because the device is
an SD card with slow reading times, on which kernel takes time to
enumerate available partitions.

Fortunately, the underlying device is back to "probing" state while
enumerating partitions. Waiting for all probing to end is enough to fix
this issue.

That's also the reason why this problem never occurs with rootwait=
parameter: the while loop inside wait_for_root() explicitly waits for
probing to be done and then the function calls async_synchronize_full().
These lines were omitted in 035641b, even though the commit says it's
based on the rootwait logic...

Anyway, calling wait_for_device_probe() after our while loop does the
job (it both waits for probing and calls async_synchronize_full).

Fixes: 035641b01e72 ("dm init: add dm-mod.waitfor to wait for asynchronously probed block devices")
Signed-off-by: Guillaume Gonnet <ggonnet.linux@gmail.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>