Reduce chance udev race with mdadm opening /dev/mdX
In some cases mdadm races with udevd with the creation of /dev/mdX and
fails to open the newly created raid array. Reduce the window by stopping
the udev exec queue while creating the raid device and resuming it
afterwards. The attached testcase no longer reproduces the issue found
only on Trusty but may still lurk in Trusty mdadm (or older).