NFSD: Zero counters when the filecache is re-initialized

[ Upstream commit 8b330f78 ]

If nfsd_file_cache_init() is called after a shutdown, be sure the
stat counters are reset.

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