[IPV4]: Thresholds in fib_trie.c are used as consts, so make them const.

There are several thresholds for trie fib hash management. They are used
in the code as a constants. Make them constants from the compiler point of
view.

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