udev rules: add watch option

udev rules: add watch option

The watch option to udev tells udev to watch our mdadm device file for
close events and on a close it rechecks the device.  This means that if,
for example, we use mdadm to --grow the array from a 4 disk to 5 disk
array, when mdadm closes the array, udev will re-read the superblock and
update its internal database with the new information.  This can also be
used to cause udev to create new device special files if, for example, a
partitioning program is used to modify the partition table on the actual
md device and that program does not call the syscall to reread the
partition table.

Signed-off-by: Doug Ledford <dledford@redhat.com>