ocfs2: rework transaction rollback in ocfs2_relink_block_group()

In ocfs2_relink_block_group(), we roll back all those changes if notify
intent to modify buffers for metadata update failed even if the relevant
buffer has not yet been modified/got dirty at that point, that are not
quite right because of:

 - None buffer has been modified/dirty if failed to call
   ocfs2_journal_access_gd() against the previous block group buffer

 - Only the previous block group buffer has got dirty if failed to call
   ocfs2_journal_access_gd() against the block group buffer

 - There is no need to roll back the change for file entry buffer at all

Those problems will not cause anything wrong but unnecessary.  This
patch fix them and kill the useless bg_ptr variable as well.

Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Cc: Younger Liu <younger.liu@huawei.com>
Cc: Sunil Mushran <sunil.mushran@gmail.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>