nl80211: fix validation of mesh path nexthop

Mesh path nexthop should be a ethernet address, but current validation
checks against 4 byte integers.

Cc: stable@vger.kernel.org
Fixes: 2ec600d6 ("nl80211/cfg80211: support for mesh, sta dumping")
Signed-off-by: Markus Theil <markus.theil@tu-ilmenau.de>
Link: https://lore.kernel.org/r/20191029093003.10355-1-markus.theil@tu-ilmenau.deSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/wireless/nl80211.c

@@ -393,7 +393,7 @@ const struct nla_policy nl80211_policy[NUM_NL80211_ATTR] = {
 	[NL80211_ATTR_MNTR_FLAGS] = { /* NLA_NESTED can't be empty */ },
 	[NL80211_ATTR_MESH_ID] = { .type = NLA_BINARY,
 				   .len = IEEE80211_MAX_MESH_ID_LEN },
-	[NL80211_ATTR_MPATH_NEXT_HOP] = { .type = NLA_U32 },
+	[NL80211_ATTR_MPATH_NEXT_HOP] = NLA_POLICY_ETH_ADDR_COMPAT,
 
 	[NL80211_ATTR_REG_ALPHA2] = { .type = NLA_STRING, .len = 2 },
 	[NL80211_ATTR_REG_RULES] = { .type = NLA_NESTED },