USB: gadget: Fix obscure lockdep violation for udc_mutex

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

USB: gadget: Fix obscure lockdep violation for udc_mutex

A recent commit expanding the scope of the udc_lock mutex in the
gadget core managed to cause an obscure and slightly bizarre lockdep
violation. In abbreviated form:

======================================================
WARNING: possible circular locking dependency detected
5.19.0-rc7+ #12510 Not tainted
------------------------------------------------------
udevadm/312 is trying to acquire lock:
ffff80000aae1058 (udc_lock){+.+.}-{3:3}, at: usb_udc_uevent+0x54/0xe0

but task is already holding lock:
ffff000002277548 (kn->active#4){++++}-{0:0}, at: kernfs_seq_start+0x34/0xe0

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #3 (kn->active#4){++++}-{0:0}:
       lock_acquire+0x68/0x84
       __kernfs_remove+0x268/0x380
       kernfs_remove_by_name_ns+0x58/0xac
       sysfs_remove_file_ns+0x18/0x24
       device_del+0x15c/0x440

-> #2 (device_links_lock){+.+.}-{3:3}:
       lock_acquire+0x68/0x84
       __mutex_lock+0x9c/0x430
       mutex_lock_nested+0x38/0x64
       device_link_remove+0x3c/0xa0
       _regulator_put.part.0+0x168/0x190
       regulator_put+0x3c/0x54
       devm_regulator_release+0x14/0x20

-> #1 (regulator_list_mutex){+.+.}-{3:3}:
       lock_acquire+0x68/0x84
       __mutex_lock+0x9c/0x430
       mutex_lock_nested+0x38/0x64
       regulator_lock_dependent+0x54/0x284
       regulator_enable+0x34/0x80
       phy_power_on+0x24/0x130
       __dwc2_lowlevel_hw_enable+0x100/0x130
       dwc2_lowlevel_hw_enable+0x18/0x40
       dwc2_hsotg_udc_start+0x6c/0x2f0
       gadget_bind_driver+0x124/0x1f4

-> #0 (udc_lock){+.+.}-{3:3}:
       __lock_acquire+0x1298/0x20cc
       lock_acquire.part.0+0xe0/0x230
       lock_acquire+0x68/0x84
       __mutex_lock+0x9c/0x430
       mutex_lock_nested+0x38/0x64
       usb_udc_uevent+0x54/0xe0

Evidently this was caused by the scope of udc_mutex being too large.
The mutex is only meant to protect udc->driver along with a few other
things. As far as I can tell, there's no reason for the mutex to be
held while the gadget core calls a gadget driver's ->bind or ->unbind
routine, or while a UDC is being started or stopped. (This accounts
for link #1 in the chain above, where the mutex is held while the
dwc2_hsotg_udc is started as part of driver probing.)

Gadget drivers' ->disconnect callbacks are problematic. Even though
usb_gadget_disconnect() will now acquire the udc_mutex, there's a
window in usb_gadget_bind_driver() between the times when the mutex is
released and the ->bind callback is invoked. If a disconnect occurred
during that window, we could call the driver's ->disconnect routine
before its ->bind routine. To prevent this from happening, it will be
necessary to prevent a UDC from connecting while it has no gadget
driver. This should be done already but it doesn't seem to be;
currently usb_gadget_connect() has no check for this. Such a check
will have to be added later.

Some degree of mutual exclusion is required in soft_connect_store(),
which can dereference udc->driver at arbitrary times since it is a
sysfs callback. The solution here is to acquire the gadget's device
lock rather than the udc_mutex. Since the driver core guarantees that
the device lock is always held during driver binding and unbinding,
this will make the accesses in soft_connect_store() mutually exclusive
with any changes to udc->driver.

Lastly, it turns out there is one place which should hold the
udc_mutex but currently does not: The function_show() routine needs
protection while it dereferences udc->driver. The missing lock and
unlock calls are added.