[IPV4]: Consolidate fib_select_default.

The difference in the implementation of the fib_select_default when
CONFIG_IP_MULTIPLE_TABLES is (not) defined looks
negligible. Consolidate it and place into fib_frontend.c.

Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>