Ch07_Memory/vmscan.bt: tracepoint:vmscan:mm_vmscan_writepage no longer available

[christian-2]

mm_vmscan_writepage is no longer available in recent kernels; vmscan.bt fails with "ERROR: tracepoint not found: vmscan:mm_vmscan_writepage" e.g. with Kernel 6.1.0-16-amd64.

Hence presumably Ch07_Memory/vmscan.bt should be refactored to make use of mm_vmscan_write_folio instead.