all: use new io.SeekFoo constants instead of os.SEEK_FOO

Automated change.

Fixes #15269

Change-Id: I8deb2ac0101d3f7c390467ceb0a1561b72edbb2f
Reviewed-on: https://go-review.googlesource.com/21962
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
diff --git a/src/net/sendfile_freebsd.go b/src/net/sendfile_freebsd.go
index f7a8529..18cbb27 100644
--- a/src/net/sendfile_freebsd.go
+++ b/src/net/sendfile_freebsd.go
@@ -53,7 +53,7 @@
 	// use the current position of the file -- if you pass it offset 0, it starts
 	// from offset 0. There's no way to tell it "start from current position", so
 	// we have to manage that explicitly.
-	pos, err := f.Seek(0, os.SEEK_CUR)
+	pos, err := f.Seek(0, io.SeekCurrent)
 	if err != nil {
 		return 0, err, false
 	}