NFSD: Clean up splice actor

A few useful observations:

 - The value in @size is never modified.

 - splice_desc.len is an unsigned int, and so is xdr_buf.page_len.
   An implicit cast to size_t is unnecessary.

 - The computation of .page_len is the same in all three arms
   of the "if" statement, so hoist it out to make it clear that
   the operation is an unconditional invariant.

The resulting function is 18 bytes shorter on my system (-Os).
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: NeilBrown <neilb@suse.de>

fs/nfsd/vfs.c

@@ -847,26 +847,21 @@ nfsd_splice_actor(struct pipe_inode_info *pipe, struct pipe_buffer *buf,
 	struct svc_rqst *rqstp = sd->u.data;
 	struct page **pp = rqstp->rq_next_page;
 	struct page *page = buf->page;
-	size_t size;
-
-	size = sd->len;
 
 	if (rqstp->rq_res.page_len == 0) {
 		get_page(page);
 		put_page(*rqstp->rq_next_page);
 		*(rqstp->rq_next_page++) = page;
 		rqstp->rq_res.page_base = buf->offset;
-		rqstp->rq_res.page_len = size;
 	} else if (page != pp[-1]) {
 		get_page(page);
 		if (*rqstp->rq_next_page)
 			put_page(*rqstp->rq_next_page);
 		*(rqstp->rq_next_page++) = page;
-		rqstp->rq_res.page_len += size;
-	} else
-		rqstp->rq_res.page_len += size;
+	}
+	rqstp->rq_res.page_len += sd->len;
 
-	return size;
+	return sd->len;
 }
 
 static int nfsd_direct_splice_actor(struct pipe_inode_info *pipe,