selftests: forwarding: lib: Split sysctl_save() out of sysctl_set()

In order to be able to save the current value of a sysctl without changing
it, split the relevant bit out of sysctl_set() into a new helper.
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20240607151357.421181-5-petrm@nvidia.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/forwarding/lib.sh

@@ -1134,12 +1134,19 @@ bridge_ageing_time_get()
 }
 
 declare -A SYSCTL_ORIG
+sysctl_save()
+{
+	local key=$1; shift
+
+	SYSCTL_ORIG[$key]=$(sysctl -n $key)
+}
+
 sysctl_set()
 {
 	local key=$1; shift
 	local value=$1; shift
 
-	SYSCTL_ORIG[$key]=$(sysctl -n $key)
+	sysctl_save "$key"
 	sysctl -qw $key="$value"
 }