librbd/cache/pwl/ssd: fix first_valid_entry calculation in retire_entries()

librbd/cache/pwl/ssd: fix first_valid_entry calculation in retire_entries()

Consider one control_block which cotain multi encode(WriteLogCacheEntry):
Log1: WriteLogEntry
Log2: WriteLogEntry
Log3: Non-WriteLogEntry
For this case, currently calc method is: control_block_pos + sizeof(control_block).
But in fact, it should: control_block_pos + sizeof(control_block) +
data_length(Log1 + Log2).

Wrong first_valid_entry will persist to superblock and restart to read.
This cause read wrong position and when decode(WriteLogCacheEntry) it
will report bug.

Fixes: https://tracker.ceph.com/issues/52323
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>