dm: free dm_io before bio_endio not after

Free the dm_io structure before calling bio_endio() instead of after it,
to ensure that the io_pool containing it is not referenced after it is
freed.

This partially fixes a problem described here
  https://www.redhat.com/archives/dm-devel/2010-February/msg00109.html

thread 1:
bio_endio(bio, io_error);
/* scheduling happens */
					thread 2:
					close the device
					remove the device
thread 1:
free_io(md, io);

Thread 2, when removing the device, sees non-empty md->io_pool (because the
io hasn't been freed by thread 1 yet) and may crash with BUG in mempool_free.
Thread 1 may also crash, when freeing into a nonexisting mempool.

To fix this we must make sure that bio_endio() is the last call and
the md structure is not accessed afterwards.

There is another bio_endio in process_barrier, but it is called from the thread
and the thread is destroyed prior to freeing the mempools, so this call is
not affected by the bug.

A similar bug exists with module unloads - the module may be unloaded
immediately after bio_endio - but that is more difficult to fix.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Alasdair G Kergon <agk@redhat.com>

drivers/md/dm.c

@@ -635,17 +635,17 @@ static void dec_pending(struct dm_io *io, int error)
 			if (!md->barrier_error && io_error != -EOPNOTSUPP)
 				md->barrier_error = io_error;
 			end_io_acct(io);
+			free_io(md, io);
 		} else {
 			end_io_acct(io);
+			free_io(md, io);
 
 			if (io_error != DM_ENDIO_REQUEUE) {
 				trace_block_bio_complete(md->queue, bio);
 
 				bio_endio(bio, io_error);
 			}
 		}
-
-		free_io(md, io);
 	}
 }