aio: fix kioctx not being freed after cancellation at exit time

The recent changes overhauling fs/aio.c introduced a bug that results in
the kioctx not being freed when outstanding kiocbs are cancelled at
exit_aio() time.  Specifically, a kiocb that is cancelled has its
completion events discarded by batch_complete_aio(), which then fails to
wake up the process stuck in free_ioctx().  Fix this by modifying the
wait_event() condition in free_ioctx() appropriately.

This patch was tested with the cancel operation in the thread based code
posted yesterday.

[akpm@linux-foundation.org: fix build]
Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Kent Overstreet <koverstreet@google.com>
Cc: Kent Overstreet <koverstreet@google.com>
Cc: Josh Boyer <jwboyer@redhat.com>
Cc: Zach Brown <zab@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/aio.c

@@ -307,7 +307,9 @@ static void free_ioctx(struct kioctx *ctx)
 	kunmap_atomic(ring);
 
 	while (atomic_read(&ctx->reqs_active) > 0) {
-		wait_event(ctx->wait, head != ctx->tail);
+		wait_event(ctx->wait,
+				head != ctx->tail ||
+				atomic_read(&ctx->reqs_active) <= 0);
 
 		avail = (head <= ctx->tail ? ctx->tail : ctx->nr_events) - head;