qed: do not evict in-use L2 table cache entries

The L2 table cache reduces QED metadata reads that would be required
when translating LBAs to offsets into the image file.  Since requests
execute in parallel it is possible to share an L2 table between multiple
requests.

There is a potential data corruption issue when an in-use L2 table is
evicted from the cache because the following situation occurs:

  1. An allocating write performs an update to L2 table "A".

  2. Another request needs L2 table "B" and causes table "A" to be
     evicted.

  3. A new read request needs L2 table "A" but it is not cached.

As a result the L2 update from #1 can overlap with the L2 fetch from #3.
We must avoid doing overlapping I/O requests here since the worst case
outcome is that the L2 fetch completes before the L2 update and yields
stale data.  In that case we would effectively discard the L2 update and
lose data clusters!

Thanks to Benoît Canet <benoit.canet@gmail.com> for extensive testing
and debugging which lead to discovery of this bug.

Reported-by: Benoît Canet <benoit.canet@gmail.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Tested-by: Benoît Canet <benoit.canet@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
