ip: fix ip zero checksum verification

ip: fix ip zero checksum verification

In one's complement, there are two representations of zero: the all
zero and the all one bit values, often referred to as +0 and -0. See
RFC 1624 section 3 for more details.
This used to be taken care of in ip4_header_checksum(), but it is no
longer the case. The check ip->checksum == ip4_header_checksum (ip) is
no longer correct in the -0 case.
Always use ip4_header_checksum_is_valid() instead (which behaves
correctly since 9a79a1ab931c3b5a7ae07d6f0fcfef7c4368a2c4).

Type: fix
Fixes: e5f0050c7a5d411f96af6401797529d58825e2af

Change-Id: Iacc6b60645a834287b085aecb9e3fdb4554cf0cf
Signed-off-by: Benoît Ganne <bganne@cisco.com>