fib_trie: Fix the description of pos and bits

1) Fix one typo: s/tn/tp/
2) Fix the description about the "u" bits.
Signed-off-by: Xunlei Pang <xlpang@redhat.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -249,7 +249,7 @@ static inline unsigned long get_index(t_key key, struct key_vector *kv)
  * index into the parent's child array. That is, they will be used to find
  * 'n' among tp's children.
  *
- * The bits from (n->pos + n->bits) to (tn->pos - 1) - "S" - are skipped bits
+ * The bits from (n->pos + n->bits) to (tp->pos - 1) - "S" - are skipped bits
  * for the node n.
  *
  * All the bits we have seen so far are significant to the node n. The rest
@@ -258,7 +258,7 @@ static inline unsigned long get_index(t_key key, struct key_vector *kv)
  * The bits from (n->pos) to (n->pos + n->bits - 1) - "C" - are the index into
  * n's child array, and will of course be different for each child.
  *
- * The rest of the bits, from 0 to (n->pos + n->bits), are completely unknown
+ * The rest of the bits, from 0 to (n->pos -1) - "u" - are completely unknown
  * at this point.
  */