locks: fix fasync_struct memory leak in lease upgrade/downgrade handling

Commit 8634b51f6c (locks: convert lease handling to file_lock_context)
introduced a regression in the handling of lease upgrade/downgrades.

In the event that we already have a lease on a file and are going to
either upgrade or downgrade it, we skip doing any list insertion or
deletion and simply re-call lm_setup on the existing lease.

As of commit 8634b51f6c however, we end up calling lm_setup on the
lease that was passed in, instead of on the existing lease. This causes
us to leak the fasync_struct that was allocated in the event that there
was not already an existing one (as it always appeared that there
wasn't one).

Fixes: 8634b51f6c (locks: convert lease handling to file_lock_context)
Reported-and-Tested-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>

fs/locks.c

@@ -1665,7 +1665,8 @@ generic_add_lease(struct file *filp, long arg, struct file_lock **flp, void **pr
 	}
 
 	if (my_fl != NULL) {
-		error = lease->fl_lmops->lm_change(my_fl, arg, &dispose);
+		lease = my_fl;
+		error = lease->fl_lmops->lm_change(lease, arg, &dispose);
 		if (error)
 			goto out;
 		goto out_setup;