LU-4924 llite: Only kill SGID/SUID bits

LU-4924 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.
Add regression test.

Lustre-commit: 1504473051df3d20e824f1064ee134501f3856e8
Lustre-change: http://review.whamcloud.com/10153

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Signed-off-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I3c8382d6f0899ce9190ccb2993af4c508942b316
Reviewed-on: http://review.whamcloud.com/10405
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>