generic/673: Add separate sgid stripping sub-tests

Even kernel doesn't get ATTR_KILL_SGID mask and get ATTR_KILL_SUID mask,
we still can strip S_ISGID mode in setattr_prepare and setattr_copy.

We should check separate sgid stripping logic whether works well
on different filesystems.

Also fix comments error.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Zorro Lang <zlang@kernel.org>