SUNRPC: remove BUG_ON calls from cache_read

Replace BUG_ON() with WARN_ON_ONCE() in two parts of cache_read().

Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>