net: check type when freeing metadata dst

Commit 3fcece12bc1b ("net: store port/representator id in metadata_dst")
added a new type field to metadata_dst, but metadata_dst_free() wasn't
updated to check it before freeing the METADATA_IP_TUNNEL specific dst
cache entry.

This is not currently causing problems since it's far enough back in the
struct to be zeroed for the only other type currently in existance
(METADATA_HW_PORT_MUX), but nevertheless it's not correct.

Fixes: 3fcece12bc1b ("net: store port/representator id in metadata_dst")
Signed-off-by: David Lamparter <equinox@diac24.net>
Cc: Jakub Kicinski <jakub.kicinski@netronome.com>
Cc: Sridhar Samudrala <sridhar.samudrala@intel.com>
Cc: Simon Horman <horms@verge.net.au>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/dst.c

@@ -299,7 +299,8 @@ EXPORT_SYMBOL_GPL(metadata_dst_alloc);
 void metadata_dst_free(struct metadata_dst *md_dst)
 {
 #ifdef CONFIG_DST_CACHE
-	dst_cache_destroy(&md_dst->u.tun_info.dst_cache);
+	if (md_dst->type == METADATA_IP_TUNNEL)
+		dst_cache_destroy(&md_dst->u.tun_info.dst_cache);
 #endif
 	kfree(md_dst);
 }