media: usb: pwc-uncompress: Use flex array destination for memcpy()

In preparation for FORTIFY_SOURCE performing run-time destination buffer
bounds checking for memcpy(), specify the destination output buffer
explicitly, instead of asking memcpy() to write past the end of what looked
like a fixed-size object.

Notice that raw_frame is a pointer to a structure that contains
flexible-array member rawframe[]:

drivers/media/usb/pwc/pwc.h:
190 struct pwc_raw_frame {
191         __le16 type;            /* type of the webcam */
192         __le16 vbandlength;     /* Size of 4 lines compressed (used by the
193                                    decompressor) */
194         __u8   cmd[4];          /* the four byte of the command (in case of
195                                    nala, only the first 3 bytes is filled) */
196         __u8   rawframe[];      /* frame_size = H / 4 * vbandlength */
197 } __packed;

Link: https://github.com/KSPP/linux/issues/200


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>