Remove the check for packets with broadcast source from if_gif's encapcheck.

The check was recommened in the draft-ietf-ngtrans-mech-05.txt. But it isn't
clear, should it compare the source with all direct broadcast addresses in the
system or not.
RFC 4213 says it is enough to verify that the source address is the address
of the encapsulator, as configured on the decapsulator. And this verification
can be extended by administrator with any other forms of IPv4 ingress filtering.

Discussed with: glebius, melifaro
Sponsored by: Yandex LLC