unix: add constants and types to work with Linux loop devices

Change-Id: I0996df39a561b50822a6e42e770d96421d6b4965
Reviewed-on: https://go-review.googlesource.com/c/sys/+/185498
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mkerrors.sh

@@ -199,6 +199,7 @@ struct ltchars {
 #include <linux/fs.h>
 #include <linux/kexec.h>
 #include <linux/keyctl.h>
+#include <linux/loop.h>
 #include <linux/magic.h>
 #include <linux/memfd.h>
 #include <linux/module.h>
@@ -435,6 +436,8 @@ ccflags="$@"
 		$2 ~ /^TC[IO](ON|OFF)$/ ||
 		$2 ~ /^IN_/ ||
 		$2 ~ /^LOCK_(SH|EX|NB|UN)$/ ||
+		$2 ~ /^LO_(KEY|NAME)_SIZE$/ ||
+		$2 ~ /^LOOP_(CLR|CTL|GET|SET)_/ ||
 		$2 ~ /^(AF|SOCK|SO|SOL|IPPROTO|IP|IPV6|ICMP6|TCP|MCAST|EVFILT|NOTE|EV|SHUT|PROT|MAP|MFD|T?PACKET|MSG|SCM|MCL|DT|MADV|PR)_/ ||
 		$2 ~ /^TP_STATUS_/ ||
 		$2 ~ /^FALLOC_/ ||