dm-pcache: validate the persisted dirty_tail chain at load

In the Linux kernel, the following vulnerability has been resolved: dm-pcache: validate the persisted dirty_tail chain at load The writeback worker follows the persisted dirty_tail chain, which is decoded from the cache device independently of the key_tail chain that cache_replay() walks and bounds. A crafted image, whose on-media fields are authenticated only by a crc32c with a fixed seed, can aim dirty_tail at a chain of last ksets that never terminates, so cache_writeback_fn() re-arms itself with no delay forever. Walk the dirty_tail chain once at load with the same hop cap cache_replay() uses and fail the table load with -EIO if it does not reach an end within n_segs hops.