nfsd: no need get cache ref when protected by rcu

rcu_read_lock/rcu_read_unlock has already provide protection for the
pointer we will reference when we call e_show. Therefore, there is no
need to obtain a cache reference to help protect cache_head.
Additionally, the .put such as expkey_put/svc_export_put will invoke
dput, which can sleep and break rcu. Stop get cache reference to fix
them all.

Fixes: ae74136 ("SUNRPC: Allow cache lookups to use RCU protection rather than the r/w spinlock")
Suggested-by: NeilBrown <neilb@suse.de>
Signed-off-by: Yang Erkun <yangerkun@huawei.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

Author: Yang Erkun

fs/nfsd/export.c

@@ -1425,13 +1425,9 @@ static int e_show(struct seq_file *m, void *p)
 		return 0;
 	}
 
-	if (!cache_get_rcu(&exp->h))
+	if (cache_check_rcu(cd, &exp->h, NULL))
 		return 0;
 
-	if (cache_check(cd, &exp->h, NULL))
-		return 0;
-
-	exp_put(exp);
 	return svc_export_show(m, cd, cp);
 }