block: clarify badblocks lifetime

block: clarify badblocks lifetime

Orabug: 22913653

The badblocks list attached to a gendisk is allocated by the driver
which equates to the driver owning the lifetime of the object.  Do not
automatically free it in del_gendisk(). This is in preparation for
expanding the use of badblocks in libnvdimm drivers and introducing
devm_init_badblocks().

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
(cherry picked from commit 20a308f09e0d29ce6f5a4114cc476a998d569bfb)
Signed-off-by: Dan Duval <dan.duval@oracle.com>