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.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Change-Id: Ic452590a6fb59010f1ee6b8b0b790557a2540d8c
Reviewed-on: http://review.whamcloud.com/10153
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>