fuse: don't dereference req->args on finished request

Move the check for async request after check for the request being already
finished and done with.

Reported-by:  <syzbot+ae0bb7aae3de6b4594e2@syzkaller.appspotmail.com>
Fixes: d4993774 ("fuse: stop copying args to fuse_req")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>