net: introduce functions to verify L4 checksums

net: introduce functions to verify L4 checksums

Since commit d5df2ae0428a ("net: fix unneeded replacement of TCP
checksum 0"), the functions rte_ipv4_udptcp_cksum() and
rte_ipv6_udptcp_cksum() can return either 0x0000 or 0xffff when used to
verify a packet containing a valid checksum.

Since these functions should be used to calculate the checksum to set in
a packet, introduce 2 new helpers for checksum verification. They return
0 if the checksum is valid in the packet.

Use this new helper in net/tap driver.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>