librbd/cache/pwl/ssd: actually use first_{valid,free}_entry on recovery

librbd/cache/pwl/ssd: actually use first_{valid,free}_entry on recovery

first_valid_entry and first_free_entry pointers are read from media
but not actually used: both m_first_valid_entry and m_first_free_entry
get assigned 0 (or garbage).  next_log_pos gets the same value as well
meaning that not only no recovery is attempted but the cache also gets
corrupted because DATA_RING_BUFFER_OFFSET is not applied.

Fixes: https://tracker.ceph.com/issues/50669
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit ef020b85fb16c1730fc08eadfd1b51d3c4cd019a)