udp: fix inner packet checksum calculation in udp-encap

udp: fix inner packet checksum calculation in udp-encap

When computing the inner packet checksum, the code wrongly
assumes that the IP version of the inner packet is the
same of the outer one. On the contrary, it is perfectly
possible to encapsulate v6 packets into v4 and viceversa,
so we need to check the IP format of the inner header before
calling vnet_calc_checksums_inline.

Ticket: VPP-2020
Type: fix

Signed-off-by: Mauro Sardara <msardara@cisco.com>
Change-Id: Ia4515563c164f6dd5096832c831a48cb0a29b3ad
Signed-off-by: Mauro Sardara <msardara@cisco.com>