net: drop_monitor: use IS_REACHABLE() to guard net_dm_hw_report()

In net/Kconfig, NET_DEVLINK implies NET_DROP_MONITOR.

The original behavior of the 'imply' keyword prevents NET_DROP_MONITOR
from being 'm' when NET_DEVLINK=y.

With the planned Kconfig change that relaxes the 'imply', the
combination of NET_DEVLINK=y and NET_DROP_MONITOR=m would be allowed.

Use IS_REACHABLE() to avoid the vmlinux link error for this case.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Neil Horman <nhorman@tuxdriver.com>

include/net/drop_monitor.h

@@ -19,7 +19,7 @@ struct net_dm_hw_metadata {
 	struct net_device *input_dev;
 };
 
-#if IS_ENABLED(CONFIG_NET_DROP_MONITOR)
+#if IS_REACHABLE(CONFIG_NET_DROP_MONITOR)
 void net_dm_hw_report(struct sk_buff *skb,
 		      const struct net_dm_hw_metadata *hw_metadata);
 #else