BACKPORT: USB: gadget: core: create sysfs link between udc and gadget

udc device and gadget device are tightly coupled, yet there's no good
way to corelate the two. Add a sysfs link in udc that points to the
corresponding gadget device.
An example use case: userspace configures a f_midi configfs driver and
bind the udc device, then it tries to locate the corresponding midi
device, which is a child device of the gadget device. The gadget device
that's associated to the udc device has to be identified in order to
index the midi device. Having a sysfs link would make things much
easier.

Signed-off-by: Roy Luo <royluo@google.com>
Link: https://lore.kernel.org/r/20240307030922.3573161-1-royluo@google.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Bug: 333778731
Change-Id: I9e3f782543eba5e026a65031aaae754daafb69ab
(cherry picked from commit 0ef40f39)
[royluo: Resolved conflict in drivers/usb/gadget/udc/core.c ]
Signed-off-by: Roy Luo <royluo@google.com>