staging: lustre: remove l_dentry define

It's a 'dentry', so use it instead of something else.

Cc: Andreas Dilger <andreas.dilger@intel.com>
Cc: Oleg Drokin <oleg.drokin@intel.com>
Cc: hpdd-discuss <hpdd-discuss@lists.01.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>