fix(lookup): resolve lower tiers outside caller namespace

[JBailes]

Summary

• resolve pool-relative lower-tier paths from SmoothFS's pinned struct path
• avoid d_path() + kern_path(), which incorrectly re-enters the calling task's mount namespace
• centralize the resolver shared by lookup, directory, and superblock paths
• add a regression harness that hides backing-tier mountpoints in a container-like mount namespace

Root cause

Plex runs inside an LXC container that sees the merged SmoothFS bind mount but intentionally does not see SmoothNAS's host-only backing-tier mountpoints. The existing resolver rendered a pinned lower path into a pathname and passed it to kern_path(). That lookup starts in the caller's mount namespace, so files present only on a lower tier returned ENOENT until a host lookup warmed the union dentry.

The fix uses vfs_path_lookup() relative to the already-pinned lower dentry and mount.

Validation

• git diff --check
• patched module builds on SmoothNAS against kernel 7.0.11-smoothkernel
• new tier_spill_mount_namespace_lookup.sh regression test fails against released SmoothFS v0.2.23 before this fix