udev: silence TOCTOU warning when creating a directory

CID#979416. There is no real race here to fix, but lets make coverity
happy and rework the code.

Note that we still fail if the directory is removed _after_ we ran
mkdir(), so the same race is still there. Coverity is complaining, though.
Rewrite the code to make it happy.

(David: rewrote the commit-message to note that this is not a race. If I'm
        wrong, blame me, not Ronny!)