[FIB]: Fix rcu_dereference() abuses in fib_trie.c

node_parent() and tnode_get_child() currently use rcu_dereference().

These functions are called from both
- readers only paths (where rcu_dereference() is needed), and
- writer path (where rcu_dereference() is not needed)

To make explicit where rcu_dereference() is really needed, I
introduced new node_parent_rcu() and tnode_get_child_rcu() functions
which use rcu_dereference(), while node_parent() and tnode_get_child()
dont use it.

Then I changed calling sites where rcu_dereference() was really needed
to call the _rcu() variants.

This should have no impact but for alpha architecture, and may help
future sparse checks.

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