ovl: fix error handling in ovl_verify_set_fh()

We hit a BUG on kfree of an ERR_PTR()...

Reported-by:  <syzbot+ff03fe05c717b82502d0@syzkaller.appspotmail.com>
Fixes: 8b88a2e6

 ("ovl: verify upper root dir matches lower root dir")
Cc: <stable@vger.kernel.org> # v4.13
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>