A singly linked list was previously used to
do fib_info object lookup for various actions
in the routing code.  This does not scale very
well with many devices and even a moderate number
of routes.  This was noted by Benjamin Lahaise.

To fix all of this we use 3 hash tables, two of
which grow dynamically as the number fib_info
objects increases while the final one is fixes in
size.

The statically sized table hashes on device index.
This is used for fib_sync_down, fib_sync_up, and
ip_fib_check_default.

The first dynamically sized table is keyed on
protocol, prefsrc, and priority.  This is used
by fib_create_info() to look for existing fib_info
objects matching the new one being constructed.

The last dynamically sized table is keyed on
the preferred source of the route if it has one
specified.  This is used by fib_sync_down when
a local address disappears.

There are still some scalability problems for
Bens test case in fib_hash.c and I will try to
attack those next.
Signed-off-by: David S. Miller <davem@davemloft.net>