drivers/staging/lustre: Fix set-but-unused whinge.

drivers/staging/lustre/lustre/fid/lproc_fid.c: In function 'ldebugfs_fid_write_common':
drivers/staging/lustre/lustre/fid/lproc_fid.c:67:6: warning: variable 'rc' set but not used [-Wunused-but-set-variable]
  int rc;

We fix it by *using* the return code to help bulletproof it.  It says it's
test code - it should be *more* bulletproof than production, not less.

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