SUNRPC: 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 c_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

net/sunrpc/cache.c

@@ -1438,17 +1438,11 @@ static int c_show(struct seq_file *m, void *p)
 		seq_printf(m, "# expiry=%lld refcnt=%d flags=%lx\n",
 			   convert_to_wallclock(cp->expiry_time),
 			   kref_read(&cp->ref), cp->flags);
-	if (!cache_get_rcu(cp))
-		return 0;
 
-	if (cache_check(cd, cp, NULL))
-		/* cache_check does a cache_put on failure */
+	if (cache_check_rcu(cd, cp, NULL))
+		seq_puts(m, "# ");
+	else if (cache_is_expired(cd, cp))
 		seq_puts(m, "# ");
-	else {
-		if (cache_is_expired(cd, cp))
-			seq_puts(m, "# ");
-		cache_put(cp, cd);
-	}
 
 	return cd->cache_show(m, cd, cp);
 }