NFS: fix the return value check by using IS_ERR

In case of error, the function rpcauth_create() returns ERR_PTR()
and never returns NULL pointer. The NULL test in the return value
check should be replaced with IS_ERR().

dpatch engine is used to auto generated this patch.
(https://github.com/weiyj/dpatch)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 1e50326..ddfebb12 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -2399,7 +2399,7 @@
 	int ret;
 
 	auth = rpcauth_create(flavor, server->client);
-	if (!auth) {
+	if (IS_ERR(auth)) {
 		ret = -EIO;
 		goto out;
 	}