media: uvcvideo: Silence memcpy() run-time false positive warnings

[ Upstream commit b839212988575c701aab4d3d9ca15e44c87e383c ]

The memcpy() in uvc_video_decode_meta() intentionally copies across the
length and flags members and into the trailing buf flexible array.
Split the copy so that the compiler can better reason about (the lack
of) buffer overflows here. Avoid the run-time false positive warning:

  memcpy: detected field-spanning write (size 12) of single field "&meta->length" at drivers/media/usb/uvc/uvc_video.c:1355 (size 1)

Additionally fix a typo in the documentation for struct uvc_meta_buf.

Reported-by:  <ionut_n2001@yahoo.com>
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216810


Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>