ocfs2: Add extra credits and access the modified bh in update_edge_lengths.

In normal tree rotation left process, we will never touch the tree
branch above subtree_index and ocfs2_extend_rotate_transaction doesn't
reserve the credits for them either.

But when we want to delete the rightmost extent block, we have to update
the rightmost records for all the rightmost branch(See
ocfs2_update_edge_lengths), so we have to allocate extra credits for them.
What's more, we have to access them also.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>