ocfs2: rollback transaction in ocfs2_group_add()

If ocfs2_journal_access_di() fails, group->bg_next_group should rollback.
Otherwise, there would be a inconsistency between group_bh and main_bm_bh.

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>