krbd: retry on an empty list from udev_enumerate_scan_devices()

krbd: retry on an empty list from udev_enumerate_scan_devices()

systemd 219 doesn't have the issue that is worked around in the
previous commit, but has a different one: udev_enumerate_scan_devices()
always succeeds, but sometimes returns an empty list when the device is
actually there.  This happens rarely and at random so I haven't been
able to get to the bottom of it yet, but it looks like another similar
race condition in libudev.

Since an empty list is expected if the device isn't there, retry just
twice with a small sleep in-between.  This appears to be enough: I got
7 occurrences per 600000 "rbd unmap" invocations, all of which needed
a single retry:

  rbd: udev enumerate missed a device, tries = 1

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>