ila: Add generic ILA translation facility

This patch implements an ILA tanslation table. This table can be
configured with identifier to locator mappings, and can be be queried
to resolve a mapping. Queries can be parameterized based on interface,
direction (incoming or outoing), and matching locator.  The table is
implemented using rhashtable and is configured via netlink (through
"ip ila .." in iproute).

The table may be used as alternative means to do do ILA tanslations
other than the lw tunnels

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>