gfs2: Clean up freeing struct gfs2_sbd

Add a free_sbd function for freeing a struct gfs2_sbd.  Use that for
freeing a super-block descriptor, either directly or via kobject_put.
Free sd_lkstats inside the kobject release function: that way,
gfs2_put_super will no longer leak sd_lkstats.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>