selftests: forwarding: lib: quote the sysctl values

When set/restore sysctl value, we should quote the value as some keys
may have multi values, e.g. net.ipv4.ping_group_range

Fixes: f5ae5778 ("selftests: forwarding: lib: Add sysctl_set(), sysctl_restore()")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Link: https://lore.kernel.org/r/20230208032110.879205-1-liuhangbin@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

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

@@ -914,14 +914,14 @@ sysctl_set()
 	local value=$1; shift
 
 	SYSCTL_ORIG[$key]=$(sysctl -n $key)
-	sysctl -qw $key=$value
+	sysctl -qw $key="$value"
 }
 
 sysctl_restore()
 {
 	local key=$1; shift
 
-	sysctl -qw $key=${SYSCTL_ORIG["$key"]}
+	sysctl -qw $key="${SYSCTL_ORIG[$key]}"
 }
 
 forwarding_enable()