ocfs2: do not call brelse() if group_bh is not initialized in ocfs2_group_add()

If group_bh is not initialized, there is no need to release.  This
problem does not cause anything wrong, but the patch would make the code
more logical.

Signed-off-by: Younger Liu <younger.liu@huawei.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Acked-by: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>