nsresourced: Verify user namespace identity on registry lookup

nsresourced: Verify user namespace identity on registry lookup

When a user namespace dies and its registry entry is torn down, the kernel
can recycle its inode number for a freshly-created namespace. A subsequent
registration or operation request can therefore find a stale registry entry
keyed by the same inode that actually belongs to a different, now-dead user
namespace.

Use NS_GET_ID to compare the kernel-assigned namespace identifier against
the stored one whenever we look up the registry from a live userns fd
(AddMount/AddControlGroup/AddNetworkInterface, plus the two registration
paths). Extract release_userns_by_info()/release_userns_by_inode() into
userns-registry.c so nsresourcework can fully clean up stale entries
(BPF allowlist, fdstore fd, cgroups, netifs, on-disk record) before reusing
the slot, and remove the now-unused userns_registry_inode_exists().

Co-developed-by: Claude Opus 4.7 <noreply@anthropic.com>