net: decnet: Fix refcount warning for new dn_fib_info

fib_treeref needs to be set after kzalloc. The old code had a ++ which
led to the confusion when the int was replaced by a refcount_t.

Fixes: 79976892 ("net: convert fib_treeref from int to refcount_t")
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20210803073739.22339-1-yajun.deng@linux.dev


Signed-off-by: Jakub Kicinski <kuba@kernel.org>