Fix owner leakage on partial chown in overlay

[doanbaotrung]

When a guest performs a partial chown to modify only the group
(e.g., chown(path, -1, gid)), the overlay allocates a new entry.
However, it previously initialized both e->uid and e->gid to the
physical host's cur_uid and cur_gid. Since new_owner was -1,
e->uid remained the physical host UID (e.g. 501). When guest later
stats the file, the overlay unconditionally applied the physical UID,
corrupting guest permissions.
Solve this by:

1. Initializing new overlay entries with (uint32_t)-1 for both
   fields to indicate "no override".
2. Only applying overrides to st_uid/st_gid in stat if the fields
   do not equal the (uint32_t)-1 sentinel.
3. Checking effective UIDs/GIDs against host values before removing
   redundant entries.

Fix #137

[cubic-dev-ai]

No issues found across 1 file

_{Re-trigger cubic}