ceph: keep leaf frag when updating fragtree

Nodes in i_fragtree are sorted according to ceph_compare_frag().
It means frag node in i_fragtree always follow its direct parent
node. To check if a leaf node is valid, we just need to check if
it's child of previous split node.

Signed-off-by: Yan, Zheng <zyan@redhat.com>