others: improve cru locking to prevent concurrent updates

There are reported cases of cron jobs being lost under heavy cru
activity (concurrent executions). Protect against concurrent add
or deletes with a more robust locking mechanism using flock.

File descriptor 278 is used as it represents the letters C-R-U
on a telephone keypad.

Ref: ac1badcd0b

release/src-rt-6.x.4708/router/others/cru

@@ -7,13 +7,8 @@ N="$F.new"
 L="/var/lock/cron.lock"
 
 if [ $# -gt 1 -a "$1" = "a" -o "$1" = "d" ]; then
-	I=$((($$ % 25) + 5))
-	while ! rm $L 2>/dev/null; do
-		I=$(($I - 1))
-		[ $I -lt 0 ] && break
-		sleep 1
-	done
-
+	{
+		flock -x 278
 		ID=$2
 		grep -v "#$ID#\$" $F >$N 2>/dev/null
 		if [ "$1" = "a" ]; then
@@ -23,8 +18,8 @@ if [ $# -gt 1 -a "$1" = "a" -o "$1" = "d" ]; then
 		fi
 		mv $N $F
 		echo root >>$U
+	} 278>$L
 
-	echo >$L
 	exit 0
 fi