block: Fix io_context leak after failure of clone with CLONE_IO

With CLONE_IO, parent's io_context->nr_tasks is incremented, but never
decremented whenever copy_process() fails afterwards, which prevents
exit_io_context() from calling IO schedulers exit functions.

Give a task_struct to exit_io_context(), and call exit_io_context() instead of
put_io_context() in copy_process() cleanup path.

Signed-off-by: Louis Rilling <louis.rilling@kerlabs.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/kernel/fork.c b/kernel/fork.c
index 166b8c4..6073534 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1310,7 +1310,8 @@
 	if (pid != &init_struct_pid)
 		free_pid(pid);
 bad_fork_cleanup_io:
-	put_io_context(p->io_context);
+	if (p->io_context)
+		exit_io_context(p);
 bad_fork_cleanup_namespaces:
 	exit_task_namespaces(p);
 bad_fork_cleanup_mm: