mac80211: mesh: handle failed alloc for rmc cache

In the unlikely case that mesh_rmc_init() fails with -ENOMEM,
the rmc pointer will be left as NULL but the interface is still
operational because ieee80211_mesh_init_sdata() is not allowed
to fail.

If this happens, we would blindly dereference rmc when checking
whether a multicast frame is in the cache.  Instead just drop the
frames in the forwarding path.
Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh.c

@@ -220,6 +220,9 @@ int mesh_rmc_check(struct ieee80211_sub_if_data *sdata,
 	u8 idx;
 	struct rmc_entry *p, *n;
 
+	if (!rmc)
+		return -1;
+
 	/* Don't care about endianness since only match matters */
 	memcpy(&seqnum, &mesh_hdr->seqnum, sizeof(mesh_hdr->seqnum));
 	idx = le32_to_cpu(mesh_hdr->seqnum) & rmc->idx_mask;