vgsp-sema: chunk the discovery scan's guest reads (64 KB) - the rebuild was starving the pump

The full-RAM discovery scan read one 4 KB page per vfio_dma_rw call:
1.5 M guest reads over the 6 GB range, ~10-11 s per rebuild, and the
rebuild runs every 10 s - so the pump got roughly one pass per rebuild.
A client whose resolutions were dropped by a session flush then waited
minutes for the scan to re-find its pages (the observed post-flush
stall: data-VA copies returning -ENOENT with the pump counters frozen).
Reading 64 KB chunks amortizes the iommu walk + pin over 16 pages; the
per-page detection logic is unchanged.

0 files changed, 0 insertions, 0 deletions