all: use SeekStart, SeekCurrent, SeekEnd

CL/19862 (f79b50b8d5bc159561c1dcf7c17e2a0db96a9a11) recently introduced the constants
SeekStart, SeekCurrent, and SeekEnd to the io package. We should use these constants
consistently throughout the code base.

Updates #15269

Change-Id: If7fcaca7676e4a51f588528f5ced28220d9639a2
Reviewed-on: https://go-review.googlesource.com/22097
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Joe Tsai <joetsai@digital-static.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/bufio/bufio_test.go b/src/bufio/bufio_test.go
index d769a6a..8580486 100644
--- a/src/bufio/bufio_test.go
+++ b/src/bufio/bufio_test.go
@@ -1475,7 +1475,7 @@
 		b.Fatal("ioutil.Discard doesn't support ReaderFrom")
 	}
 	for i := 0; i < b.N; i++ {
-		r.Seek(0, 0)
+		r.Seek(0, io.SeekStart)
 		srcReader.Reset(onlyReader{r})
 		n, err := srcReader.WriteTo(ioutil.Discard)
 		if err != nil {