usb: gadget: f_uac1_legacy: validate control request size

usb: gadget: f_uac1_legacy: validate control request size

f_audio_complete() copies req->length bytes into a 4-byte stack
variable:

  u32 data = 0;
  memcpy(&data, req->buf, req->length);

req->length is derived from the host-controlled USB request path,
which can lead to a stack out-of-bounds write.

Validate req->actual against the expected payload size for the
supported control selectors and decode only the expected amount
of data.

This avoids copying a host-influenced length into a fixed-size
stack object.

Signed-off-by: Taegu Ha <hataegu0826@gmail.com>
Cc: stable <stable@kernel.org>
Link: https://patch.msgid.link/20260401191311.3604898-1-hataegu0826@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>