fix extended partition detection

fix extended partition detection

# mdadm --detail --export /dev/md127p1

Before:
MD_LEVEL=raid5
MD_DEVICES=4
MD_METADATA=0.90

After:
MD_LEVEL=raid5
MD_DEVICES=4
MD_CONTAINER=/dev/md0
MD_MEMBER=0
MD_UUID=55746a20:925d24a7:4f9bd7e2:9c9a411f

We parse the symlink target with a format:

../../block/mdXXX/mdXXXpYY

...and need the second '/' from the end of the string to read detect a
'md' device.

Reported-by: Krzysztof Wasilewski <krzysztof.wasilewski@intel.com>
Cc: Przemyslaw Czarnowski <przemyslaw.hawrylewicz.czarnowski@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>