drivers/staging/lustre: Clean up another C warnining: set but not used

drivers/staging/lustre/lustre/fid/../include/lustre_cfg.h: In function 'lustre_cfg_free':
drivers/staging/lustre/lustre/fid/../include/lustre_cfg.h:253:6: warning: variable 'len' set but not used [-Wunused-but-set-variable]
  int len;

Yep, we're just gonna call kfree, no need to calculate len. Bye-bye.

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