[IPV4]: Compact some ifdefs in the fib code.

There are places that check for CONFIG_IP_MULTIPLE_TABLES
twice in the same file, but the internals of these #ifdefs
can be merged.

As a side effect - remove one ifdef from inside a function.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>