vxlan: ICMP error lookup handler

Export an encap_err_lookup() operation to match an ICMP error against a
valid VNI.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>