usbip: vudc: fix NULL deref in vep_dequeue()

usbip: vudc: fix NULL deref in vep_dequeue()

vep_alloc_request() wasn't initializing vrequest->udc, so cancellations
on the FunctionFS AIO path were arriving in vep_dequeue without a valid
UDC reference.

Since vrequest->udc is never actually properly used anywhere, we opt to
remove it, and update vep_dequeue to obtain a reference to the udc with
ep_to_vudc(), consistent with the other vep_ ops.

AFAICT this bug has existed for ~10 years. Seems that nobody has really
stressed the FunctionFS AIO path on usbip's vudc.

I tested this fix in a QEMU aarch64 guest driving FunctionFS endpoints
via AIO. Before the fix, running `usbip attach` from the host would
cause the guest to oops with the following backtrace:

Call trace:
 vep_dequeue+0x1c/0xe4 (P)
 usb_ep_dequeue+0x14/0x20
 ffs_aio_cancel+0x24/0x34
 __arm64_sys_io_cancel+0xb0/0x124
 do_el0_svc+0x68/0x100
 el0_svc+0x18/0x5c
 el0t_64_sync_handler+0x98/0xdc
 el0t_64_sync+0x154/0x158

Assisted-by: opencode:openai/gpt-5.5
Cc: stable <stable@kernel.org>
Fixes: b6a0ca111867 ("usbip: vudc: Add UDC specific ops")
Reviewed-by: Igor Kotrasinski <i.kotrasinsk@samsung.com>
Signed-off-by: Sam Day <me@samcday.com>
Link: https://patch.msgid.link/20260626-usbip-vudc-deque-fix-v3-1-98c2dc4d6a48@samcday.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>