librbd/cache/pwl/ssd/WriteLog: don't crash on split log entries

librbd/cache/pwl/ssd/WriteLog: don't crash on split log entries

write_log_entries() will split a log entry at the end of the log, the
remainder is written to the beginning at DATA_RING_BUFFER_OFFSET.  On
the read side aio_read_data_block() doesn't handle this case and just
crashes.  Unless the workload in use is <= 4K, the image is rendered
unusable sooner or later.

Fixes: https://tracker.ceph.com/issues/50589
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>