webdav: replace os.SEEK_XXX with io.SeekXXX

Change-Id: I304410ad143377d48bd6eafcbf2413dcfc4e70a2
GitHub-Last-Rev: 0136a8a0c9d9af31a6f3f71cafc374f2800b9a4d
GitHub-Pull-Request: golang/net#151
Reviewed-on: https://go-review.googlesource.com/c/net/+/432257
Run-TryBot: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
diff --git a/webdav/file.go b/webdav/file.go
index c48a17e..3cd19ff 100644
--- a/webdav/file.go
+++ b/webdav/file.go
@@ -547,11 +547,11 @@
 	npos := f.pos
 	// TODO: How to handle offsets greater than the size of system int?
 	switch whence {
-	case os.SEEK_SET:
+	case io.SeekStart:
 		npos = int(offset)
-	case os.SEEK_CUR:
+	case io.SeekCurrent:
 		npos += int(offset)
-	case os.SEEK_END:
+	case io.SeekEnd:
 		npos = len(f.n.data) + int(offset)
 	default:
 		npos = -1