dpll: fix dpll_pin_on_pin_register() for multiple parent pins

In the Linux kernel, the following vulnerability has been resolved:

dpll: fix dpll_pin_on_pin_register() for multiple parent pins

In scenario where pin is registered with multiple parent pins via
dpll_pin_on_pin_register(..), all belonging to the same dpll device.
A second call to dpll_pin_on_pin_unregister(..) would cause a call trace,
as it tries to use already released registration resources (due to fix
introduced in b446631f355e). In this scenario pin was registered twice,
so resources are not yet expected to be release until each registered
pin/pin pair is unregistered.

Currently, the following crash/call trace is produced when ice driver is
removed on the system with installed E810T NIC which includes dpll device:

WARNING: CPU: 51 PID: 9155 at drivers/dpll/dpll_core.c:809 dpll_pin_ops+0x20/0x30
RIP: 0010:dpll_pin_ops+0x20/0x30
Call Trace:
? __warn+0x7f/0x130
? dpll_pin_ops+0x20/0x30
dpll_msg_add_pin_freq+0x37/0x1d0
dpll_cmd_pin_get_one+0x1c0/0x400
? __nlmsg_put+0x63/0x80
dpll_pin_event_send+0x93/0x140
dpll_pin_on_pin_unregister+0x3f/0x100
ice_dpll_deinit_pins+0xa1/0x230 [ice]
ice_remove+0xf1/0x210 [ice]

Fix by adding a parent pointer as a cookie when creating a registration,
also when searching for it. For the regular pins pass NULL, this allows to
create separated registration for each parent the pin is registered with.