unix: add missing const (MTDFILEMODE) from <mtd/mtd-user.h>

CL 318211 added constants from <mtd/mtd-user.h>. The regex was
too strict and as a result MTDFILEMODE was not included.

This CL relaxes the regex so that MTDFILEMODE is included.

Fixes golang/go#46448

Change-Id: I2667d1690782cf8ce98b26cb6305e7386cbd0d46
Reviewed-on: https://go-review.googlesource.com/c/sys/+/323391
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
diff --git a/unix/zerrors_linux_amd64.go b/unix/zerrors_linux_amd64.go
index aec8754..75730cc 100644
--- a/unix/zerrors_linux_amd64.go
+++ b/unix/zerrors_linux_amd64.go
@@ -137,6 +137,7 @@
 	MEMSETBADBLOCK                   = 0x40084d0c
 	MEMUNLOCK                        = 0x40084d06
 	MEMWRITEOOB                      = 0xc0104d03
+	MTDFILEMODE                      = 0x4d13
 	NFDBITS                          = 0x40
 	NLDLY                            = 0x100
 	NOFLSH                           = 0x80