poib: assign link-local address according to RFC

RFC 4391 specifies that the IB interface GID should be re-used as IPv6
link-local address. Since the code in in6_get_hw_ifid() ignored
IFT_INFINIBAND case, ibX interfaces ended up with the local address
borrowed from some other interface, which is non-compliant.

Use lowest eight bytes from GID for filling the link-local address,
same as Linux.

Reviewed by: bz (previous version), ae, hselasky, slavash,
Sponsored by: Mellanox Technologies
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20006