unix: add BLK* Linux ioctls

These are useful for re-reading partition tables and getting device
sizes.

Fixes golang/go#19370

Change-Id: Ia8dbe3416a48dbdbceafa0fff0131d71057d97ca
Reviewed-on: https://go-review.googlesource.com/37640
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/mkerrors.sh

@@ -353,7 +353,8 @@ ccflags="$@"
 		$2 ~ /^GRND_/ ||
 		$2 ~ /^SPLICE_/ ||
 		$2 !~ "WMESGLEN" &&
-		$2 ~ /^W[A-Z0-9]+$/ {printf("\t%s = C.%s\n", $2, $2)}
+		$2 ~ /^W[A-Z0-9]+$/ ||
+		$2 ~ /^BLK/ {printf("\t%s = C.%s\n", $2, $2)}
 		$2 ~ /^__WCOREFLAG$/ {next}
 		$2 ~ /^__W[A-Z0-9]+$/ {printf("\t%s = C.%s\n", substr($2,3), $2)}