include/linux/radix-tree.h: fix error in docs about locks

This text refers to the "first 7 functions", which was correct when
written but became incorrect when Johannes Weiner added another function
to the list in 139e5616 ("lib: radix_tree: tree node interface").

Change the text to correctly refer to the first 8 functions.
Signed-off-by: Adam Barth <aurorean@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/radix-tree.h

@@ -154,7 +154,7 @@ do {									\
  * radix_tree_gang_lookup_tag_slot
  * radix_tree_tagged
  *
- * The first 7 functions are able to be called locklessly, using RCU. The
+ * The first 8 functions are able to be called locklessly, using RCU. The
  * caller must ensure calls to these functions are made within rcu_read_lock()
  * regions. Other readers (lock-free or otherwise) and modifications may be
  * running concurrently.