Provide a worked example for utilising the lx_radix_tree_lookup function

Link: http://lkml.kernel.org/r/e786008ac5aec4b84198812805b326d718bdeb4b.1462865983.git.jan.kiszka@siemens.comSigned-off-by: NKieran Bingham <kieran.bingham@linaro.org>
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>