[IPV4]: fix IP_FIB_HASH kconfig warning

This patch fixes the following kconfig warning:
  net/ipv4/Kconfig:92:warning: defaults for choice values not supported

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>