[WIP] feat(kernel): Control shared mount propagation during boot - #949

[Blocked - do not merge]

Blocking this until the stability concerns below are resolved. Stability comes first; this hook touches mount flags on every mount during early boot, so it needs to be provably safe on every supported kernel.

Must fix

1. Fallback struct mount is unsafe. If fs/mount.h is not found, the file falls back to a hand-copied struct mount and then writes dest_mnt->mnt.mnt_flags. The layout differs between kernel versions, so a wrong offset silently corrupts kernel memory. Fail the build (or compile the feature out) when the header is unavailable. Nothing else in kernel/ does this.
2. Probe argument assumptions are unverified. PT_REGS_PARM1/2 are assumed to be source_mnt / dest_mnt. The signature of attach_recursive_mnt changed in newer kernels, and the function may be inlined (so symbol_name fails to resolve). Please list the kernel versions / GKI branches this was actually tested on, and make it a no-op (not an error) where the symbol or signature does not match.
3. Module loop device capture. "First /dev/block/loop* seen" can be an APEX loop mount rather than the KernelSU modules image, which would make the post-post-fs-data selective mode never match. Please show on a device that the captured name is the modules device.
4. Scope before post-fs-data. Until ksu_zygote_started is set, every shared destination mount in the system is made private for the duration of the call, including Android's own early mounts. Please describe why this does not break propagation that init/apexd rely on, with test results.

Should fix

• ksu_set_zygote_started() is called from on_post_fs_data(), when Zygote has not started yet. Rename or move.
• Use strscpy instead of strncpy (removed from the tree in #3529).
• Rate-limit or drop the per-mount pr_info spam.
• Follow the repo's formatting (4 spaces, clang-format).
• ksu_mount_propagation_registered is a plain bool shared between on_boot_completed() and module exit.

Not built or run on a device by me; the points above come from reading the diff.