Fix ambiguous 6.1 sample

FFmpeg (as well as Libav) have two layouts called "6.1":
AV_CH_LAYOUT_6POINT1 and AV_CH_LAYOUT_6POINT1_BACK. We call them "6.1"
and "6.1(back)".

Also pass an explicit layout to lavc instead of the name. This allows
generating a file with the ambiguous layout.