Improve sceAtracGetBufferInfoForResetting()

This fixes the Ys Seven issue, at least for me.  The `atrac->first.fileoffset` value was way higher than the game was expecting.

This doesn't complete pass the test from hrydgard/pspautotests#108, but most of the numbers match up now.

Be warned: I can't really hear anything to tell if it breaks music, though.  Might be good for someone to check that.

-[Unknown]