drivers/staging/lustre: Fix another C compiler whine: set but not used

  CC [M]  drivers/staging/lustre/lustre/libcfs/module.o
drivers/staging/lustre/lustre/libcfs/module.c: In function 'lustre_insert_debugfs':
drivers/staging/lustre/lustre/libcfs/module.c:670:17: warning: variable 'entry' set but not used [-Wunused-but-set-variable]
  struct dentry *entry;
                     ^

Just ignore the dentry returned, and add a comment that we *know*
we're not really leaking the dentry because something else will be able
to reap it via recursion.

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>