Improve ability to lookup primary device associated with a partition

Improve ability to lookup primary device associated with a partition

Improve lib/device/device.c:_primary_dev()'s ability to look up the
primary device associated with all partitions; including blkext
(e.g. partitions directly on MD).  The same will also work for obscure
sysfs paths; e.g.: paths with mangled names like the HP cciss driver
uses: /sys/block/cciss!c0d0/cciss!c0d0p1/

Signed-off-by: Mike Snitzer <snitzer@redhat.com>