sunrpc: clean cache_detail immediately when flush is written frequently

We will write /proc/net/rpc/xxx/flush if we want to clean cache_detail.
This updates nextcheck to the current time and calls cache_flush -->
cache_clean to clean cache_detail.
If we write this interface again within one second, it will only increase
flush_time and nextcheck without actually cleaning cache_detail.
Therefore, if we keep writing this interface repeatedly within one second,
flush_time and nextcheck will keep increasing, even far exceeding the
current time, making it impossible to clear cache_detail through the flush
interface or cache_cleaner.
If someone frequently calls the flush interface, we should immediately
clean the corresponding cache_detail instead of continuously accumulating
nextcheck.

Signed-off-by: Li Lingfeng <lilingfeng3@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>