net/nat: Unlock on outbound NAT entry creation failure.

Release nat_lock before returning -ENOENT from the IPv4 and IPv6
outbound NAT paths when NAT entry creation fails.

Signed-off-by: Shunchao Hu <ankohuu@gmail.com>

net/nat/ipv4_nat.c

@@ -795,6 +795,7 @@ int ipv4_nat_outbound(FAR struct net_driver_s *dev,
         {
           /* Outbound entry creation failed, should have entry. */
 
+          nat_unlock();
           return -ENOENT;
         }
     }

net/nat/ipv6_nat.c

@@ -683,6 +683,7 @@ int ipv6_nat_outbound(FAR struct net_driver_s *dev,
         {
           /* Outbound entry creation failed, should have entry. */
 
+          nat_unlock();
           return -ENOENT;
         }
     }