nsresourced: re-link GID delegation file after atomic UID file write

nsresourced: re-link GID delegation file after atomic UID file write

userns_registry_remove() restores a sub-delegated UID range by writing
the previous owner's data to u<UID>.delegate with WRITE_STRING_FILE_ATOMIC.
Atomic writes go via a temp file and rename, which replaces the directory
entry with a fresh inode and severs the hardlink to g<GID>.delegate. The
stale GID side then keeps pointing at the prior inode with outdated owner
and ancestor data, so subsequent lookups via GID return wrong results.

Re-create the hardlink after the atomic write so the two views stay in
sync, matching what userns_registry_store() already does after writing
a new delegation.