ovl: fix recursive oi->lock in ovl_link()

linking a non-copied-up file into a non-copied-up parent results in a nested call to mutex_lock_interruptible(&oi->lock). Fix this by copying up target parent before ovl_nlink_start(), same as done in ovl_rename().

~/unionmount-testsuite$ ./run --ov -s ~/unionmount-testsuite$ ln /mnt/a/foo100 /mnt/a/dir100/

WARNING: possible recursive locking detected
--------------------------------------------
ln/1545 is trying to acquire lock:
00000000bcce7c4c (&ovl_i_lock_key[depth]){+.+.}, at:

     ovl_copy_up_start+0x28/0x7d
 but task is already holding lock:
 0000000026d73d5b (&ovl_i_lock_key[depth]){+.+.}, at:
     ovl_nlink_start+0x3c/0xc1

---
 fs/overlayfs/dir.c | 4 ++++
 1 file changed, 4 insertions(+)

index 276914ae3c60..e1a55ecb7aba 100644
@@ -663,6 +663,10 @@ static int ovl_link(struct dentry *old, struct inode *newdir,
 	if (err)
 		goto out_drop_write;
 
+	err = ovl_copy_up(new->d_parent);
+	if (err)
+		goto out_drop_write;
+
 	if (ovl_is_metacopy_dentry(old)) {
 		err = ovl_set_redirect(old, false);
 		if (err)

2.17.1