[GFS2] Given device ID rather than s_id in "id" sysfs file

This patch changes the /sys/fs/gfs2/<s_id>/id file to give the device
id "major:minor" rather than the s_id.  That enables gfs2_tool to
match devices properly (by id, not name) when locating the tuning files.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>