gstpad: use hook_id instead of hook in called_probes list

gstpad: use hook_id instead of hook in called_probes list

A pointer to a hook in this list can easily not be unique, given both
the slice-allocator reusing memory, and the OS re-using freed blocks
in malloc.

By doing many repeated add and remove of probes, this becomes very easily
reproduced.

Instead use hook_id, which *is* unique for a added GHook.