net: fix inconsistent error values on Read

This change fixes inconsistent error values on Read,
ReadFrom{,UDP,IP,Unix} and ReadMsg{UDP,IP,Unix}.

Updates #4856.

Change-Id: I7de5663094e09be2d78cdb18ce6f1e7ec260888d
Reviewed-on: https://go-review.googlesource.com/8992
Reviewed-by: Ian Lance Taylor <iant@golang.org>
diff --git a/src/net/sendfile_freebsd.go b/src/net/sendfile_freebsd.go
index ffc1472..a0324a3 100644
--- a/src/net/sendfile_freebsd.go
+++ b/src/net/sendfile_freebsd.go
@@ -92,7 +92,7 @@
 			// This includes syscall.ENOSYS (no kernel
 			// support) and syscall.EINVAL (fd types which
 			// don't implement sendfile together)
-			err = &OpError{"sendfile", c.net, c.raddr, err1}
+			err = err1
 			break
 		}
 	}