staging/lustre/llite: Add exception entry check after radix_tree

We need to check for an exception entry after radix_tree lookup.

Signed-off-by: Yang Sheng <yang.sheng@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/10709
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5162


Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: James Simmons <uja.ornl@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>