fib_trie: Prevent allocating tnode if bits is too big for size_t

This patch adds code to prevent us from attempting to allocate a tnode with
a size larger than what can be represented by size_t.

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>