runsc/boot: activate journaling uppers on checkpoint restore - #17611

Prepare journaled rootfs uppers and hide self-backed filestores before
restored tasks start running, then activate their journals in
postRestore once background page loading completes.

Stack

1. pgalloc: support adopting existing pages and unsaved reservations #17592 — pgalloc: support adopting existing pages and unsaved reservations (+531 / -3)
2. tmpfs/journal: add journal header format, Create, and Close #17593 — tmpfs/journal: add journal header format, Create, and Close (+510 / -0)
3. tmpfs/journal: add Open header validation and page adoption #17594 — tmpfs/journal: add Open header validation and page adoption (+331 / -5)
4. tmpfs/journal: add record schema, State types, and SnapshotSizer #17595 — tmpfs/journal: add record schema, State types, and SnapshotSizer (+289 / -17)
5. tmpfs/journal: add binary record encoding and Snapshot writer #17596 — tmpfs/journal: add binary record encoding and Snapshot writer (+493 / -7)
6. tmpfs/journal: decode and replay snapshot records #17597 — tmpfs/journal: decode and replay snapshot records (+487 / -3)
7. tmpfs/journal: add live mutation writers and delta replay #17598 — tmpfs/journal: add live mutation writers and delta replay (+469 / -2)
8. tmpfs/journal: add incomplete journal state and recovery/validation tests #17599 — tmpfs/journal: add incomplete journal state and recovery/validation tests (+445 / -0)
9. tmpfs/journal: expose FilestoreEnd high-water mark for shrinking #17600 — tmpfs/journal: expose FilestoreEnd high-water mark for shrinking (+321 / -2)
10. tmpfs: rebuild filesystem state from a replayed journal #17601 — tmpfs: rebuild filesystem state from a replayed journal (+452 / -1)
11. tmpfs: add journal snapshot compaction and release lifecycle #17602 — tmpfs: add journal snapshot compaction and release lifecycle (+444 / -43)
12. tmpfs: journal live namespace, inode, and extent mutations #17603 — tmpfs: journal live namespace, inode, and extent mutations (+398 / -3)
13. tmpfs: add background compactor, save/restore hooks, and self-whiteout sync #17604 — tmpfs: add background compactor, save/restore hooks, and self-whiteout sync (+258 / -1)
14. tmpfs,overlay: support activating journaling on populated filesystems #17605 — tmpfs,overlay: support activating journaling on populated filesystems (+492 / -1)
15. runsc/config: add journaling rootfs upper flags and policy enums #17606 — runsc/config: add journaling rootfs upper flags and policy enums (+161 / -0)
16. runsc/specutils: add journaling rootfs upper annotations and resolution #17607 — runsc/specutils: add journaling rootfs upper annotations and resolution (+401 / -2)
17. runsc/container: manage, lock, and shrink journaled rootfs filestores #17608 — runsc/container: manage, lock, and shrink journaled rootfs filestores (+403 / -7)
18. runsc/boot: wire fresh and imported journaling rootfs uppers #17609 — runsc/boot: wire fresh and imported journaling rootfs uppers (+483 / -43)
19. runsc/boot: activate journaling uppers populated by tar or fs-restore #17610 — runsc/boot: activate journaling uppers populated by tar or fs-restore (+500 / -4)
20. runsc/boot: activate journaling uppers on checkpoint restore #17611 — runsc/boot: activate journaling uppers on checkpoint restore (+516 / -2) (this PR)
21. test/syscalls,g3doc: add journaled overlay test variants and user guide #17153 — test/syscalls,g3doc: add journaled overlay test variants and user guide (+195 / -2)