dhcp: fix sd_dhcp_client_set_client_id() for infiniband addresses

dhcp: fix sd_dhcp_client_set_client_id() for infiniband addresses

Infiniband addresses are 20 bytes (INFINIBAND_ALEN), but only the last
8 bytes are suitable for putting into the client-id.

This bug had no effect for networkd, because sd_dhcp_client_set_client_id()
has only one caller which always uses ARPHRD_ETHER type.

I was unable to find good references for why this is correct ([1]). Fedora/RHEL
has patches for ISC dhclient that also only use the last 8 bytes ([2], [3]).
RFC 4390 (Dynamic Host Configuration Protocol (DHCP) over InfiniBand) [4] does
not discuss the content of the client-id either.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1658057#c29
[2] https://bugzilla.redhat.com/show_bug.cgi?id=660681
[3] https://src.fedoraproject.org/rpms/dhcp/blob/3ccf3c8d815df4b8e11e1a04850975f099273d5d/f/dhcp-lpf-ib.patch
[4] https://tools.ietf.org/html/rfc4390