nfsd: fix NULL dereference in nfs3svc_encode_getaclres

In error cases the dentry may be NULL.

Before 20798dfe24, the encoder also checked dentry and
d_really_is_positive(dentry), but that looks like overkill to me--zero
status should be enough to guarantee a positive dentry.

This isn't the first time we've seen an error-case NULL dereference
hidden in the initialization of a local variable in an xdr encoder.  But
I went back through the other recent rewrites and didn't spot any
similar bugs.

Reported-by: JianHong Yin <jiyin@redhat.com>
Reviewed-by: Chuck Lever III <chuck.lever@oracle.com>
Fixes: 20798dfe24 ("NFSD: Update the NFSv3 GETACL result encoder...")
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs3acl.c

@@ -172,7 +172,7 @@ static int nfs3svc_encode_getaclres(struct svc_rqst *rqstp, __be32 *p)
 	struct nfsd3_getaclres *resp = rqstp->rq_resp;
 	struct dentry *dentry = resp->fh.fh_dentry;
 	struct kvec *head = rqstp->rq_res.head;
-	struct inode *inode = d_inode(dentry);
+	struct inode *inode;
 	unsigned int base;
 	int n;
 	int w;
@@ -181,6 +181,7 @@ static int nfs3svc_encode_getaclres(struct svc_rqst *rqstp, __be32 *p)
 		return 0;
 	switch (resp->status) {
 	case nfs_ok:
+		inode = d_inode(dentry);
 		if (!svcxdr_encode_post_op_attr(rqstp, xdr, &resp->fh))
 			return 0;
 		if (xdr_stream_encode_u32(xdr, resp->mask) < 0)