vgsp-sema: backfill a promoted pool semaphore page

r1640: UVM's channel-pool release was written to the boot-cache page
(pa=0x2600000) and 54 ms later a verified VAS walk promoted a different
page (pa=0x103ac0000, sysmem) to primary.  The value was never carried
over, so the guest tracker polling the walk page read 0 and GZDoom spun
forever in uvm_channel_manager_create -> uvm_push_end_and_wait ->
uvm_spin_loop.

On every primary/secondary swap in svc_pool_boot_store(), max-merge the
old primary page into the new primary page (semaphore words are
forward-only monotonic counters; read both pages first, never write on
a read failure, never merge a page into itself) and log pool-merge.

0 files changed, 0 insertions, 0 deletions