[GFS2] fs/gfs2/ops_fstype.c:fill_super_meta(): fix NULL dereference

Don't dereference new->s_root when we do know it's NULL.

Spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>