[dev.link] cmd/link: stop Seek file in munmap

If mmap is used, we don't do file I/O anymore, so no need to Seek
in the file either.

Change-Id: Ic8350eb66404398420df18560f6a7acbb3fcf7e3
Reviewed-on: https://go-review.googlesource.com/c/go/+/240398
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
diff --git a/src/cmd/link/internal/ld/outbuf_mmap.go b/src/cmd/link/internal/ld/outbuf_mmap.go
index 41c436e..7280027 100644
--- a/src/cmd/link/internal/ld/outbuf_mmap.go
+++ b/src/cmd/link/internal/ld/outbuf_mmap.go
@@ -38,8 +38,4 @@
 	}
 	syscall.Munmap(out.buf)
 	out.buf = nil
-	_, err := out.f.Seek(out.off, 0)
-	if err != nil {
-		Exitf("seek output file failed: %v", err)
-	}
 }