LU-4015 ldiskfs: access external journal using device path

LU-4015 ldiskfs: access external journal using device path

Currently ldiskfs when configured with an external journal
uses the device number to access it. The problem with this
approach is that the device number can change across
reboots. This patch adds a new mount option -o journal_path
to tell ldiskfs where the external journal is instead.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I43fff89c2bebeff4f0305cdc24cefa534fb9fe67
Reviewed-on: http://review.whamcloud.com/7777
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>