staging/lustre/llite: Only kill SGID/SUID bits

Check that attr mode is valid before using it when determining if to
clear SGID and SUID bits in ll_setattr.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-on: http://review.whamcloud.com/10153
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4924


Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>