common: allow dmflakey tests to work without udev

If udev is not present, we need to run "dmsetup mknodes" to make sure
/dev/mapper/flakey-test is created or destroyed as appropriate.  On a
system with udev, running "dmsetup mknodes" will be a no-op.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>