sunrpc: add helpers to count and snapshot pending cache requests

sunrpc: add helpers to count and snapshot pending cache requests

Add sunrpc_cache_requests_count() and sunrpc_cache_requests_snapshot()
to allow callers to count and snapshot the pending upcall request list
without exposing struct cache_request outside of cache.c.

Both functions skip entries that no longer have CACHE_PENDING set.

The snapshot function takes a cache_get() reference on each item so the
caller can safely use them after the queue_lock is released.

These will be used by the nfsd generic netlink dumpit handler for
svc_export upcall requests.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>