NFSD: Use xdr_inline_decode() to decode NFSv3 symlinks

Replace the check for buffer over/underflow with a helper that is
commonly used for this purpose. The helper also sets xdr->nwords
correctly after successfully linearizing the symlink argument into
the stream's scratch buffer.
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs3xdr.c

@@ -616,8 +616,6 @@ nfs3svc_decode_symlinkargs(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 {
 	struct nfsd3_symlinkargs *args = rqstp->rq_argp;
 	struct kvec *head = rqstp->rq_arg.head;
-	struct kvec *tail = rqstp->rq_arg.tail;
-	size_t remaining;
 
 	if (!svcxdr_decode_diropargs3(xdr, &args->ffh, &args->fname, &args->flen))
 		return false;
@@ -626,16 +624,10 @@ nfs3svc_decode_symlinkargs(struct svc_rqst *rqstp, struct xdr_stream *xdr)
 	if (xdr_stream_decode_u32(xdr, &args->tlen) < 0)
 		return false;
 
-	/* request sanity */
-	remaining = head->iov_len + rqstp->rq_arg.page_len + tail->iov_len;
-	remaining -= xdr_stream_pos(xdr);
-	if (remaining < xdr_align_size(args->tlen))
-		return false;
-
-	args->first.iov_base = xdr->p;
+	/* symlink_data */
 	args->first.iov_len = head->iov_len - xdr_stream_pos(xdr);
-
-	return true;
+	args->first.iov_base = xdr_inline_decode(xdr, args->tlen);
+	return args->first.iov_base != NULL;
 }
 
 bool