Robbie Williamson found some bugs in the mandatory locking implementation.
This patch fixes all the problems he found:

 - Fix null pointer dereference caused by sys_truncate() passing a null filp.
 - Honour the O_NONBLOCK flag when calling ftruncate()
 - Local variable `fl' wasn't being initialised correctly in
   locks_mandatory_area()
 - Don't return -ENOLCK from __posix_lock_file() when FL_ACCESS is set.