[IPV4] fib_trie: removes a memset() call in tnode_new()

tnode_alloc() already clears allocated memory, using kcalloc() or
alloc_pages(GFP_KERNEL|__GFP_ZERO, ...)

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>