ipsec: enable ipv6 udp checksum offload

ipsec: enable ipv6 udp checksum offload

RFC6935 section 5 states that, by default, the UDP checksum must be
computed when originating an IPv6 UDP packet. The default behavior
may be overridden when conditions defined by RFC6936 are satisfied.
However this implementation does not satisfy all the conditions so
the checksum must be computed.

After ESP encryption the packet is an IPv6 UDP packet so set the
l3_hdr_offset and l4_hdr_offset values, and set the UDP_CKSUM
offload flag in the buffer.

Type: improvement

Co-authored-by: Cian Ferriter <[email protected]>
Change-Id: I9f8c434c9fe9dbddd8890d5ae366984bfcf34067
Signed-off-by: Jeff Shaw <[email protected]>