usb: gadget: udc: core: Fix segfault if udc_bind_to_driver() for pending driver fails

If a gadget driver is in the pending drivers list, a UDC
becomes available and udc_bind_to_driver() fails, then it
gets deleted from the pending list.
i.e. list_del(&driver->pending) in check_pending_gadget_drivers().

Then if that gadget driver is unregistered,
usb_gadget_unregister_driver() does a list_del(&driver->pending)
again thus causing a page fault as that list entry has been poisoned
by the previous list_del().

Fix this by using list_del_init() instead of list_del() in
check_pending_gadget_drivers().

Test case:

- Make sure no UDC is available
- modprobe g_mass_storage file=wrongfile
- Load UDC driver so it becomes available
	lun0: unable to open backing file: wrongfile
- modprobe -r g_mass_storage

[   60.900431] Unable to handle kernel paging request at virtual address dead000000000108
[   60.908346] Mem abort info:
[   60.911145]   ESR = 0x96000044
[   60.914227]   Exception class = DABT (current EL), IL = 32 bits
[   60.920162]   SET = 0, FnV = 0
[   60.923217]   EA = 0, S1PTW = 0
[   60.926354] Data abort info:
[   60.929228]   ISV = 0, ISS = 0x00000044
[   60.933058]   CM = 0, WnR = 1
[   60.936011] [dead000000000108] address between user and kernel address ranges
[   60.943136] Internal error: Oops: 96000044 [#1] PREEMPT SMP
[   60.948691] Modules linked in: g_mass_storage(-) usb_f_mass_storage libcomposite xhci_plat_hcd xhci_hcd usbcore ti_am335x_adc kfifo_buf omap_rng cdns3 rng_core udc_core crc32_ce xfrm_user crct10dif_ce snd_so6
[   60.993995] Process modprobe (pid: 834, stack limit = 0x00000000c2aebc69)
[   61.000765] CPU: 0 PID: 834 Comm: modprobe Not tainted 4.19.59-01963-g065f42a60499 #92
[   61.008658] Hardware name: Texas Instruments SoC (DT)
[   61.014472] pstate: 60000005 (nZCv daif -PAN -UAO)
[   61.019253] pc : usb_gadget_unregister_driver+0x7c/0x108 [udc_core]
[   61.025503] lr : usb_gadget_unregister_driver+0x30/0x108 [udc_core]
[   61.031750] sp : ffff00001338fda0
[   61.035049] x29: ffff00001338fda0 x28: ffff800846d40000
[   61.040346] x27: 0000000000000000 x26: 0000000000000000
[   61.045642] x25: 0000000056000000 x24: 0000000000000800
[   61.050938] x23: ffff000008d7b0d0 x22: ffff0000088b07c8
[   61.056234] x21: ffff000001100000 x20: ffff000002020260
[   61.061530] x19: ffff0000010ffd28 x18: 0000000000000000
[   61.066825] x17: 0000000000000000 x16: 0000000000000000
[   61.072121] x15: 0000000000000000 x14: 0000000000000000
[   61.077417] x13: ffff000000000000 x12: ffffffffffffffff
[   61.082712] x11: 0000000000000030 x10: 7f7f7f7f7f7f7f7f
[   61.088008] x9 : fefefefefefefeff x8 : 0000000000000000
[   61.093304] x7 : ffffffffffffffff x6 : 000000000000ffff
[   61.098599] x5 : 8080000000000000 x4 : 0000000000000000
[   61.103895] x3 : ffff000001100020 x2 : ffff800846d40000
[   61.109190] x1 : dead000000000100 x0 : dead000000000200
[   61.114486] Call trace:
[   61.116922]  usb_gadget_unregister_driver+0x7c/0x108 [udc_core]
[   61.122828]  usb_composite_unregister+0x10/0x18 [libcomposite]
[   61.128643]  msg_cleanup+0x18/0xfce0 [g_mass_storage]
[   61.133682]  __arm64_sys_delete_module+0x17c/0x1f0
[   61.138458]  el0_svc_common+0x90/0x158
[   61.142192]  el0_svc_handler+0x2c/0x80
[   61.145926]  el0_svc+0x8/0xc
[   61.148794] Code: eb03003f d10be033 54ffff21 a94d0281 (f9000420)
[   61.154869] ---[ end trace afb22e9b637bd9a7 ]---
Segmentation fault

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/udc/core.c

@@ -1143,7 +1143,7 @@ static int check_pending_gadget_drivers(struct usb_udc *udc)
 						dev_name(&udc->dev)) == 0) {
 			ret = udc_bind_to_driver(udc, driver);
 			if (ret != -EPROBE_DEFER)
-				list_del(&driver->pending);
+				list_del_init(&driver->pending);
 			break;
 		}