rhashtable: unexport and make rht_obj() static

No need to export rht_obj(), all inner to outer object translations
occur internally. It was intended to be used with rht_for_each() which
now primarily serves as the iterator for rhashtable_remove_pprev() to
effectively flush and free the full table.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>