GFS2: Fix sgid propagation when using ACLs

This cleans up the mode setting code when creating inodes. The
SGID bit was being reset by setattr_copy() when the user creating a
subdirectory was not in the owning group. When ACLs are in use this
SGID bit should have been propagated if the ACL allows creation of
a subdirectory. GFS2's behaviour now matches that of the other ACL
supporting filesystems in this regard.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>