xfrm: Support crypto offload for inbound IPv4 UDP-encapsulated ESP packet

If xfrm_input() is called with UDP_ENCAP_ESPINUDP, the packet is
already processed in UDP layer that removes the UDP header.
Therefore, there should be no much difference to treat it as an
ESP packet in the XFRM stack.

Test: Enabled dir=in IPsec crypto offload, and verified IPv4
      UDP-encapsulated ESP packets on both wifi/cellular network
Signed-off-by: Mike Yu <yumike@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>