[IPV4]: Unify access to the routing tables.

Replace the direct pointers to local and main tables with
calls to fib_get_table() with appropriate argument.

This doesn't introduce additional dereferences, but makes the access to fib
tables uniform in any (CONFIG_IP_MULTIPLE_TABLES) case.

Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Acked-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>