librbd/cache/pwl/ssd: move finish_op() to the end of callback function

librbd/cache/pwl/ssd: move finish_op() to the end of callback function

finish_op() of ssd cache is not in the end of callback function in
append_op_log_entries(), and after finish_op(),  some operation also
need to get m_lock. So, during shutdown, wait_for_ops() thinks all OPs
are over, and no thread will acquire the m_lock, In the subsequent
operation of shutdown, the m_lock is obtained, and _aio_stop() in
bdev->close() waits for all aio_writes() and aio_submit() to end
when the m_lock is held, but the callback function of aio_write() is
waiting for the m_lock, causing a deadlock. Move finish_op() to the
end to fix dead lock.

Fixes: https://tracker.ceph.com/issues/52235
Signed-off-by: Yin Congmin <congmin.yin@intel.com>