nfsd: update comment over __nfsd_file_cache_purge

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

Author: jtlayton

fs/nfsd/filecache.c

@@ -907,7 +907,8 @@ nfsd_file_cache_init(void)
  * @net: net-namespace to shut down the cache (may be NULL)
  *
  * Walk the nfsd_file cache and close out any that match @net. If @net is NULL,
- * then close out everything. Called when an nfsd instance is being shut down.
+ * then close out everything. Called when an nfsd instance is being shut down,
+ * and when the exports table is flushed.
  */
 static void
 __nfsd_file_cache_purge(struct net *net)