inputselector: Avoid deadlock when requesting pads
The deadlock was the following:
- One thread requests a new pad, the internal lock is kept while adding the pad
- Another thread (or the same one) requests the internal links of a pad (could be that pad)... which also requires that lock.
That internal lock is not required when adding the pad to the element (which is the last action when requesting a new pad). The fact it will be actually used will be after the request pad function is released.