nfsd: make symbol nfsd_reply_cache_shrinker static

symbol 'nfsd_reply_cache_shrinker' only used within this file. It should
be static.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>