rhashtable: Add rhashtable_next_key() API

rhashtable: Add rhashtable_next_key() API

Introduce a simpler iteration mechanism for rhashtable that lets
the caller continue from an arbitrary position by supplying the
previous key, without the per-iterator state of the
rhashtable_walk_* API.

  void *rhashtable_next_key(struct rhashtable *ht,
                            const void *prev_key);

Caller holds RCU; passes NULL prev_key for the first element or
the previously returned key to advance. Walks tbl->future_tbl
chain so in-flight rehashes are observed.

Best-effort: in case of concurrent resize, provides no guarantees:
 - may produce duplicate elements
 - may skip any amount of elements
 - termination of the loop is not guaranteed in case of
 sustained rehash. Callers are advised to bound loop externally
 or avoid inserting new elements during such loop.

Returns ERR_PTR(-ENOENT) if prev_key is not found.
Behavior on tables with duplicate keys is undefined.
rhltable is not supported — returns ERR_PTR(-EOPNOTSUPP).

Signed-off-by: Mykyta Yatsenko <yatsenko@meta.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Link: https://lore.kernel.org/r/20260605-rhash-v7-1-5b8e05f8630d@meta.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>