tests/00readonly: Run udevadm settle before setting ro

tests/00readonly: Run udevadm settle before setting ro

In some recent kernel versions, 00readonly fails with:

  mdadm: failed to set readonly for /dev/md0: Device or resource busy
  ERROR: array is not read-only!

This was traced down to a race condition with udev holding a reference
to the block device at the same time as trying to set it read only.

To fix this, call udevadm settle before setting the array read only.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>