Allow IP over IB to work with multiple FIBs.

Call M_SETFIB() to make sure the IPoIB packet is directed to the correct
interface-specific FIB.

This was sufficient to allow general-purpose routing using the default FIB,
and a separate FIB for routing between IPoIB on ib0 and IPoEthernet on mce0.

Reviewed by: hselasky
Obtained from: Anmol Kumar <anmolk at panasas dot com>
MFC after: 1 week
Sponsored by: Panasas
Differential Revision: https://reviews.freebsd.org/D25239