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>